surpress warning when MRB_DISABLE_STDIO

Fix SEGV when unshared env

Should clear method name

- Fix method name in top-level
- Fix SEGV when call Exception#backtrace if callinfo over CALLINFO_INIT_SIZE(32)

In mrbgem_spec.rake, added support for @mrblib_dir @objs_dir, so that a gem can have custom source directory names

Fix mrbc_filename leak

In mrbgem_spec.rake, added support for @mrblib_dir @objs_dir, so that a gem can have custom source directory names

instance_eval env should remake Fix #3191

Add new test for optimize VM

To avoid VM nesting with mrb_funcall()

parse.y: lparen_arg statement

allow parenthesised statement as a method argument.
reported [Feature:12686] in bugs.ruby-lang.org fixed in CRuby 2.4.

Fix String#ord failure which return a negative value

Correct method name

parse.y: fix chained assignments

fix syntax errors with chained assignment with op assign.
reported [Bug:12669] in bugs.ruby-lang.org fixed in CRuby 2.4.

parse.y: simplify 90e8ce5c

simplify tOP_ASGN rules by command_rhs and arg_rhs rules with
%prec.

reported in [Bug:12402] in bugs.ruby-lang.org fixed in CRuby 2.4

despite the fact original authors agreed to distribute their work
under MIT license, it does not mean mt19937ar.[ch] became the work
of mruby developers. To clarify, we updated copyright and license
notice of the source files.

make mrb_hash_values() a public API function

intptr_t should be used instead of uint32_t to hold a pointer.

Fix warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

Update mirb.c

Reduce needless Array generation in some String methods

Here are some benchmarks:

each_char:

    # /tmp/each_char.rb
    a = "a" * 1000000
    a.each_char do |x|
    end

Without this change:

    % time bin/mruby /tmp/each_char.rb
    bin/mruby /tmp/each_char.rb  1.07s user 0.02s system 99% cpu 1.088 total

With this change:

    % time bin/mruby /tmp/each_char.rb
    bin/mruby /tmp/each_char.rb  0.52s user 0.01s system 99% cpu 0.530 total

2 times faster with this change.

codepoints:

    # /tmp/codepoints.rb
    a = "a" * 1000000
    a.codepoints do |x|
    end

Without this change:

    % time bin/mruby /tmp/codepoints.rb
    bin/mruby /tmp/codepoints.rb  1.16s user 0.05s system 99% cpu 1.216 total

With this change:

    % time bin/mruby /tmp/codepoints.rb
    bin/mruby /tmp/codepoints.rb  0.56s user 0.02s system 99% cpu 0.589 total