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

Fix Class#new call to inherited and add a couple Class tests

Adding line numbers to the output of mirb.

Tests and another fix for issue #1467

Remove debug printf

Before:

    > "hi"
    hi
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 1: unterminated string meets end of file

After

    > "hi"
    hi
    > d
    (mirb):2: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):3: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 5: unterminated string meets end of file