Reduce warning for enumeration

22 enumeration values not handled in switch: 'MRB_TT_FALSE',
'MRB_TT_FREE', 'MRB_TT_TRUE'... [-Wswitch]

Sacrifice some micro-optimization to remove duplicate code

* Filter `MRB_TT_ENV` and `MRB_TT_ICLASS`.
* Set `mrb->string_class` in `mrb_init_exception` instead.

Fix typo in doc/api/mruby.h.md

Document rest mrbconfs.

Implement Proc#parameters

Update mrbconf.

OP_ENTER use MBR_ASPEC_* macros

MRB_ASPEC_REST() is macro just only checks whether truthy or falsy.
And truthy means exist 1 rest argument in current proc.

Proc#arity return 0 when opcode was not OP_ENTER

Not create local variables in scope_new

When `n2->car` is 0 and `n2->cdd` is not 0,
unnecessary comma was printed before first local variables.

add a few const qualifier

Fix to not print unnecessary comma.