Fix build on MIPS of linux

Added some documentation for some of the functions found in array.h.

MIPS of Linux platform is supported frexpl(3).
This fixes to use the frexpl that are provided with gcc if user wants to build
on MIPS of Linux platform.
Signe-doff-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

remove unnecessary mrb_immediate_p()

directly call ary_new_capa()

push only after OP_GETCONST in VAL mode

`!mrb_array_p(ary2)` and `mrb_type(obj) != MRB_TT_DATA` are  sufficient.

Compiler codes is moved to "mruby-compiler".
Executable `mrbc` is moved to "mruby-bin-mrbc".

only generate gets of non-local vars in VAL mode

fix two potential cases of signed integer overflow

Added more documentation for some of the functions found in mruby.h and value.h

This fixes a crash for code like "#{@a;1}".

Unlike CRuby globals are excluded too since mruby doesn't implement
hooking.

Wrote documentation for functions found in the value.h file.

Added documentation of more of the C API functions found inside mruby.h

fix masgn nosplat array rhs bug

The rest lhs variable has to be an empty array if rhs is an array with
less elements than pre + post lhs variables. The codegen generated
OP_ARRAY with an invalid length (such as 127 for *a, b = []) because rn
was negative.