Update ConvSym to 2.12.1 (symbol heap overflow fix)

[vladikcomper]

This PR updates ConvSym from 2.12 to 2.12.1. A minor patch fixes an edge-case bug in symbol generation where if one block is too large and symbol heap exceeds 64 kb, all further blocks are skipped. It also improves ConvSym README and adds some formatting improvements.

Fix background

If symbol generation pushed to absolute limit, where combined symbol texts for a small 64kb slice of ROM exceed 64kb even in compressed form, ConvSym gracefully displays an error like this:

ERROR: Symbols heap for block 01 exceeded 64kb limit, no more symbols can be stored in this block.

Block 01 in the example covers offsets 0x010000..0x01FFFF (second 64kb of ROM). The error means symbol table container ran out of space to store symbols across this small 64kb slice/block at the beginning of the ROM.

This error isn't critical: some symbols will be lost, but symbol table itself is perfectly valid.

However, there was an overlooked bug, where this error caused all further blocks to be ignored, meaning symbol table will be cut after the first heap overflow.

[vladikcomper]

Windows build of ConvSym is significantly smaller now. It appears the previous was either built -Og (it's now -O2) or I didn't strip debug information. It's still bigger than Linux build (~200 kb), because this one is statically linked.

[Stephane-D]

Thanks for the very reactive / quick fix 👍