Remove duplicated `String#each_char`

Add customized `mrb_ro_data_p()`

User definable `mrb_ro_data_p()` functions are available
by defining `MRB_USE_CUSTOM_RO_DATA_P`.

(Limitation) It can not be defined as an inline function.

Singleton class of frozen object should be frozen

Before this patch:

  p (class << Object.new.freeze; self end).frozen?                #=> false
  sc = class << (o=Object.new); self end; o.freeze; p sc.frozen?  #=> false

After this patch / Ruby:

  p (class << Object.new.freeze; self end).frozen?                #=> true
  sc = class << (o=Object.new); self end; o.freeze; p sc.frozen?  #=> true

Fix modiying class variable to frozen class/module

Fix "ambiguous first argument" warning in `test/t/float.rb`

Add `assert_match` and `assert_not_match`

Change modifier to `MRB_INLINE` from `static inline`

Use `MRB_ASPEC_XXX()` macro in `codedump()`

Commented out `String#scan` because it is not implemented yet

Expand `BOXWORD_SET_VALUE()` macro in `include/mruby/boxing_word.h`

In `SET_OBJ_VALUE()`, branch isn't removed because `switch` condition isn't
constant expression.

Fixes the twiddle wakka comparison algorithm

Add type check (conversion) in `String#[]=`

Add `mrb_true_p()` and `mrb_false_p()` macro functions

Before this patch:

  'a'[0] = 1        #=> 1
  'a'[:a] = '1'     #=> ArgumentError
  'a'[:a, 0] = '1'  #=> ArgumentError
  'a'[0, :a] = '1'  #=> ArgumentError
  'a'[0, 1] = 1     #=> 1

After this patch / Ruby:

  'a'[0] = 1        #=> TypeError
  'a'[:a] = '1'     #=> TypeError
  'a'[:a, 0] = '1'  #=> TypeError
  'a'[0, :a] = '1'  #=> TypeError
  'a'[0, 1] = 1     #=> TypeError

Remove duplicated `include Comparable` in `mrblib/string.rb`

Add `Array#sample` test

And simplify tests for `Array#shuffle` and `Array#shuffle!`.

Avoid potential zero size array declaration; fix #4382

Fix test in mruby-io

Fix missing assertions in `mruby-random` test