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

Support to call without block to String#each_char

Fix broken msvc build

Fix crash in mrb_parser_free() due to freed memory access

yard docs for string.h

Should not define to `Struct` class

reported by https://gist.github.com/miura1729/53fbd8af889c289a79108e38635b2378
fix proposed by @miura1729 in
https://github.com/miura1729/mruby/commit/b1b7933f7aa950cfb747b06327a0d0340f3e4ff8

refactored redundant functions; removed optimized accessors for safety
this fix also close #3178

String#insert should be destructive

Should raise LocalJumpError when no block given

C doesn't have try-catch

Fix warning shift-negative-value

Fix compilation error with GC_PROFILE.

Fix public methods

Update doc, write about add_test_dependency

when multiple mrb_gc_register() were called for the same object

need to find out real memory bug that appears in full-debug/mrbtest

add function for checked mrb_int multiplication