Add Array#delete_if
Use boolean macro in is_valid_c_symbol_name()
use `stack_type` instead of raw `unsigned int`
allow method definition in cmdarg

save cmdarg_stack and isolate command argument state from outer scope
to allow method definition in cmdarg

import from ruby/ruby@04bb9d6b75a55d4000700769eead5a5cb942c25b
Merge pull request #2094 from suzukaze/add-array.delete_if

Add Array#delete_if
Merge pull request #2095 from suzukaze/refactor-dump.c

Use boolean macro in is_valid_c_symbol_name()
Merge pull request #2096 from nobu/def_in_cmdarg

Def in cmdarg
fix optional arguments in rhs

define optional arguments as argument variables in the rhs default
expressions, as same as mere assignment expressions.

Import ruby/ruby@01740f0c273c89f7bcff3d5014d73c8ff6fb1986
Merge pull request #2097 from nobu/optional_arguments_in_rhs

fix optional arguments in rhs
 Add Array#reject_bang
Merge pull request #2098 from suzukaze/add-array.reject_bang

 Add Array#reject!
adjust indent
fix optional block arguments in rhs

define optional block arguments as argument variables in the rhs
default expressions, as same as mere assignment expressions.

Import ruby/ruby@01740f0c273c89f7bcff3d5014d73c8ff6fb1986
Merge pull request #2099 from nobu/optional_arguments_in_rhs

Optional arguments in rhs
tests should not depend on `mruby-proc-ext`
Merge pull request #2100 from cremno/fix-tests

tests should not depend on `mruby-proc-ext`
Remove `Struct` defined check in test.
Add document to Fiber#transfer .
Remove unnecessary `require 'fiber'` note in Fiber.current document.
Merge pull request #2102 from take-cheeze/fiber_doc

Improve mruby-fiber document.