Remove DISABLE_REGEXP

Changed debugger hook to disable

Added a hook at VM code fetch.

Pluggable regexp

add assert_{true,nil,equal} methods.

Conflicts:
	src/class.c

Fix mirb crash 

String#gsub fix with last character

Add Module#remove_const Test Case

fix incorrect output for Module#remove_const

support.

Extend MiniRake::DSL module to prevent inheritance pollution

```include MiniRake::DSL``` on toplevel will pollute ancestor tree of every object.

```extend MiniRake::DSL``` will add those methods only to ```main``` object.

Rake does the same: https://github.com/jimweirich/rake/blob/master/lib/rake/dsl_definition.rb#L153

Added String#sub/sub! and String#gsub/gsub!

Fix typos in compilation guide

string contains null byte check utility function (mrb_str_to_cstr)