Remove function delarations not implemented in code

Crash if you call String#start_with/end_with with non-String arguments.

Conflicts:
	mrbgems/mruby-string-ext/src/string.c
	mrbgems/mruby-string-ext/test/string.rb

Bug in String#end_with? resulting from incorrect length determination

Bug in String#start_with? resulting from incorrect length determination

support file extension .cpp/.cxx in mrbgem and tool

Fix a problem of 'z' option and shared string

add x86_64-darwin host platform.

android doesn't have log2

remove a unnecessary return

Return when block argument error

redesign mruby/data.h API; use DATA_PTR() for raw data pointer, DATA_GET_PTR() to type safe retrieval (TypeError will be raised), DATA_CHECK_GET_PTR() to get nil if type mismatched