Add test of $0 value in bin/mruby related to #2103 .

OP_SEND refactoring

remove `mrb_str_buf_cat`

Also set $0 for binary files

It does the same as `mrb_str_cat`.

Remove `Struct` defined check in test.

Improve mruby-fiber document.

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 .