Fix to not print unnecessary comma.

When `n2->car` is 0 and `n2->cdd` is not 0,
unnecessary comma was printed before first local variables.
Add `Symbol#intern`.
Merge pull request #2418 from take-cheeze/sym_intern

Add `Symbol#intern`.
revert #2344 because of #2395
Merge pull request #2333 from yui-knk/fix/dump

Fix to not print unnecessary comma.
Merge branch 'master' of github.com:mruby/mruby
add a few const qualifier
Not create local variables in scope_new
Merge pull request #2419 from santazhang/use-const

add a few const qualifier
Merge pull request #2420 from ksss/reduce-lv

Not create local variables in scope_new
Proc#arity return 0 when opcode was not OP_ENTER
Merge pull request #2421 from ksss/safe-arity

Proc#arity return 0 when opcode was not OP_ENTER
proc.h: MRB_ASPEC_REST() macro return 1 or 0

MRB_ASPEC_REST() is macro just only checks whether truthy or falsy.
And truthy means exist 1 rest argument in current proc.
OP_ENTER: Rewrite split aspec use macros
Merge pull request #2422 from ksss/aspec-macro

OP_ENTER use MBR_ASPEC_* macros