tests should not depend on `mruby-proc-ext`

Optional 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

 Add Array#reject!

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

Def in cmdarg

Use boolean macro in is_valid_c_symbol_name()

Add Array#delete_if

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

import from ruby/ruby@04bb9d6b75a55d4000700769eead5a5cb942c25b

do not reuse deleted entry. 

Suppress test script syntax warning.

Implement Struct#size and Struct#length .

Implement Symbol#casecmp

String#casecmp should be call `to_str`

Symbol class include Comparable module.

Implement :checksum_hash mrbgem option

Add Rake command 'deep_clean' which cleans everything including the mrbg...

Can I add me to AUTHORS?

Fix typos in tests for mruby-string-utf8

when non String object sent
And should be raise TypeError
when can not responsed to `to_str`

This allows to use a mgem with a specific checksum hash.
The following modification were necessary to implement this
function:
  - add run_checkout build command to use 'git checkout'
  - skip shallow copy in case checksum_hash is used
  - make 'master' the default branch if branch isn't defined

Use boolean macro instead of integer literal in `is_debug_info_defined`.