Support local variables information dumping.
Implement Kernel.local_variables for testing dumped local variables information.
Remove `lv_len` and use `nlocals - 1` instead.
Check that `lv`'s length is always `nlocals - 1`.
Merge branch 'dump_lv' of https://github.com/take-cheeze/mruby into take-cheeze-dump_lv
rename lv section header from LOCV to LVAR
resize register number in LVAR section from 32bits to 16bits
Merge branch 'take-cheeze-dump_lv'