There is a lot potential in compress bytecodes and make more use of the
coding space.  This patch introduces "multi" bytecodes which have their
argument included in the bytecode (by addition).

UNARY_OP and BINARY_OP now no longer take a 1 byte argument for the
opcode.  Rather, the opcode is included in the first byte itself.

LOAD_FAST_[0,1,2] and STORE_FAST_[0,1,2] are removed in favour of their
multi versions, which can take an argument between 0 and 15 inclusive.
The majority of LOAD_FAST/STORE_FAST codes fit in this range and so this
saves a byte for each of these.

LOAD_CONST_SMALL_INT_MULTI is used to load small ints between -16 and 47
inclusive.  Such ints are quite common and now only need 1 byte to
store, and now have much faster decoding.

In all this patch saves about 2% RAM for typically bytecode (1.8% on
64-bit test, 2.5% on pyboard test).  It also reduces the binary size
(because bytecodes are simplified) and doesn't harm performance.