Saves 50-100 bytes of code.

On 32-bit archs this makes the scope_t struct 48 bytes in size, which fits
in 3 GC blocks (previously it used 4 GC blocks).  This will lead to some
savings when compiling scripts because there are usually quite a few scopes,
one for each function and class.

Note that qstrs will fit in 16 bits, this assumption is made in a few other
places.

Generates slightly smaller and more efficient code.

Utility functions for keyboard interrupt handling, to be reused across
(baremetal) ports.

The memory read/write I2C functions now take an optional keyword-only
parameter that specifies the number of bits in the memory address.
Only mem-addrs that are a multiple of 8-bits are supported (otherwise
the behaviour is undefined).

Due to the integer type used for the address, for values larger than 32
bits, only 32 bits of address will be sent, and the rest will be padded
with 0s. Right now no exception is raised when that happens. For values
smaller than 8, no address is sent. Also no exception then.

Tested with a VL6180 sensor, which has 16-bit register addresses.

Due to code refactoring, this patch reduces stmhal and esp8266 builds
by about 50 bytes.

There is now just the exception instance on the stack when an exception is
raised, not the full (type, exc, traceback).

Following how other objects work, set/frozenset methods should use the
mp_check_self() macro to check the type of the self argument, because in
most cases this check can be a null operation.

Saves about 100-180 bytes of code for builds with set and frozenset
enabled.

It's simpler and improves code coverage.

The native emitter/compiler restricts viper functions to 4 args, so there
is no need for an extra check in the dynamic dispatch.

To reset the flags we should write to the single bit only, not the entire
register (otherwise all other settings in the register are cleared).

Fixes #2457.

Having a micropython.const identity function, and writing "from micropython
import const" at the start of scripts that use the const feature, allows to
write scripts which are compatible with CPython, and with uPy builds that
don't include const optimisation.

This patch adds such a function and updates the tests to do the import.

This corresponds to the change in the way exception values are stored on
the Python value stack.

When an exception is raised and is to be handled by the VM, it is stored
on the Python value stack so the bytecode can access it.  CPython stores
3 objects on the stack for each exception: exc type, exc instance and
traceback.  uPy followed this approach, but it turns out not to be
necessary.  Instead, it is enough to store just the exception instance on
the Python value stack.  The only place where the 3 values are needed
explicitly is for the __exit__ handler of a with-statement context, but
for these cases the 3 values can be extracted from the single exception
instance.

This patch removes the need to store 3 values on the stack, and instead
just stores the exception instance.

Code size is reduced by about 50-100 bytes, the compiler and VM are
slightly simpler, generate bytecode is smaller (by 2 bytes for each try
block), and the Python value stack is reduced in size for functions that
handle exceptions.

Fixes for more pedantic warnings.

This uses extension introduced in CPython 3.5: if wbits (dictionary size
code) has value 16 + 8..15, it means that gzip-formatted stream expected.

This fixes constant substitution so that only standalone identifiers are
replaced with their constant value (if they have one).  I.e. don't
replace NAME in expressions like obj.NAME or NAME = expr.

LIST_APPEND, MAP_ADD and SET_ADD have been removed, and STORE_COMP has
been added in adaf0d86.

qstrs ids are restricted to fit within 2 bytes already (eg in persistent
bytecode) so it's safe to use a uint16_t to store them in mp_arg_t.  And
the flags member only needs a maximum of 2 bytes so can also use uint16_t.

Savings in code size can be significant when many mp_arg_t structs are
used for argument parsing.  Eg, this patch reduces stmhal by 480 bytes.

In CPython, module-level .wrap_socket() function actually doesn't accept
(or document) this param, only SSLContext.wrap_socket() has.

When the clock is too fast for the i2c slave, it can temporarily hold
down the scl line to signal to the master that it needs to wait. The
master should check the scl line when it is releasing it after
transmitting data, and wait for it to be released.

This change has been tested with a logic analyzer and an i2c slace
implemented on an atmega328p using its twi peripheral, clocked at 8Mhz.
Without the change, the i2c communication works up to aboy 150kHz
frequency, and above that results in the slave stuck in an unresponsive
state. With this change, communication has been tested to work up to
400kHz.