compiler flag -Ygen-asmp to emit .asmp textual files for bytecode

This commit adds the -Ygen-asmp compiler flag, similar in purpose to what -Ygen-javap used to be.

The ASM-based counterpart to javap is useful even in those cases where javap is available,
as it produces for example readable (textual) output for pickles.

Additionally, javap displays quite differently two constant pools that have identical contents
(javap shows their physical layout, not just the information the entries hold).

Finally, stack maps (classfile version 50 and up) are displayed in encoded form by javap, their expansion by ASM is more readable.
BytecodeWriters.BytecodeWriter doesn't need to fiddle with Symbol

This commit allows a BytecodeWriter to focus on File, Paths, and streams. GenASM now does the conversion from Symbol to File.
Merge pull request #2459 from magarciaEPFL/backendish7

similar in purpose to -Ygen-javap but with some advantages: -Ygen-asmp