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.

Add `Symbol#intern`.

Start documenting C APIs.

Add mrbconf document.

Reduce new string creation in `capitalize`, `downcase`, `upcase` method of `Symbol` class extension.
Add test to check unmodified result is working.