When writing PUSH instructions, obey requested opcode

Previously fonttools was choosing between the optimal opcode
from PUSHB, NPUSHB, PUSHW, and NPUSHW.  We now respect
whatever was requested and err if the format doesn't support
the data.

Morevoer, if the number of numbers to push is more than 255,
we add multiple push instructions to push all the numbers.

Finally, add a new pseudo-opcode "PUSH" that automatically
chooses the best format.

This, by itself, reduces roundtrip noise by not optimizing
the bytecode as it was before.  In a followup commit I'll
change the bytecode disassembler to always produce PUSH
in the textual instructions instead of the four variants.
That way, we get both the optimization during assembling,
and reduced noise in XML.

Part of https://github.com/behdad/fonttools/issues/73
Generate PUSH mnemonic for PUSHB/NPUSHB/PUSHW/NPUSHW

Reduces XML noise while reenabling the optimization
during assembly.

Fixes https://github.com/behdad/fonttools/issues/73
When disassembling, merge multiple PUSH ops

Fonts typically have multiple consecutive PUSHB / PUSHW
operators.  Merge them in the disassembled output.

This, for now, generates worse assmebly because all PUSHB's
grouped with PUSHW's now generate as PUSHW.  Followup fix
will address that.
Choose most compact representation when writing PUSH instructions

Can use some testing...