A GC in stmhal port now only scans true root pointers, not entire BSS.
This reduces base GC time from 1700ms to 900ms.

This is a simple optimisation inspired by JITing technology: we cache in
the bytecode (using 1 byte) the offset of the last successful lookup in
a map. This allows us next time round to check in that location in the
hash table (mp_map_t) for the desired entry, and if it's there use that
entry straight away.  Otherwise fallback to a normal map lookup.

Works for LOAD_NAME, LOAD_GLOBAL, LOAD_ATTR and STORE_ATTR opcodes.

On a few tests it gives >90% cache hit and greatly improves speed of
code.

Disabled by default.  Enabled for unix and stmhal ports.

This patch consolidates all global variables in py/ core into one place,
in a global structure.  Root pointers are all located together to make
GC tracing easier and more efficient.

Addresses issue #1044 (see also #1040).  Could do with a better fix.

This is consistent with how BC_JUMP was handled before. We never show jumps
destinations relative to jump instrucion itself, only relative to beginning
of function. Another useful way to show them as absolute (real memory
address), and this change makes result expected and consistent with how
BC_JUMP is shown.

Confirmed that it improves perfomance of simple "for i in range(N): pass"
loop by 15% on Core2.

Remove some duplication in the code for generating
qstrdefs.generated.h and py-version.h

The compiler treats `if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE)` as
a normal statement and generates assembly for it in degug mode as if MICROPY_ERROR_REPORTING
is an actual symbol instead of a preprocessor definition.
As such linking fails because mp_arg_error_terse_mismatch is not defined when
MICROPY_ERROR_REPORTING_TERSE is detailed or normal.

- Use a single file env.props for defining the main directories used when building.
  env.props resolves the base directory and defines overridable output directories,
  and is used by all other build files.
- Fix the build currently failing, basically because the preprocessing command for generating
  qstrdefs uses different include directories than the build itself does.
  (specifically, qstrdefs.h uses #include "py/mpconfig.h" since the fixes for #1022
  in 51dfcb4b, so we need to use the base directory as include directory, not the py dir itself).
  So define a single variable containing the include directories instead and use it where needed.

This reduces dependency on assembler, and allows to consolidate global
variables in the future.

Addresses issue #1022.

Add a command for converting the WAV files to the amp skin tutorial, so that people can use their own files easily.

We are not word-for-word compatible with CPython exceptions, so we are
free to make them short but informative in order to reduce code size.
Also, try to make messages the same as existing ones where possible.

Reduces code size when MICROPY_ERROR_REPORTING_TERSE is selected.