Move `NilClass#to_h` to `mruby-object-ext` from `mruby-enum-ext`

Refine `Array#(permutation|combination) test`

- No guarantees about the order in which the permutations/combinations
  are yielded.

- Drop dependency on `Enumerator`.

When the size of Xorshift128 seed (`sizeof(uint32)*4`) is bigger than
ISTRUCT_DATA_SIZE, `Random` uses Xorshift96 instead.

Add encoding argument to `Integral#chr`

Currently, `Integral#chr` in mruby changes behavior by `MRB_UTF8_STRING`
setting.

before this patch:

  $ bin/mruby -e 'p 171.chr'  #=> "\xab"  (`MRB_UTF8_STRING` is disabled)
  $ bin/mruby -e 'p 171.chr'  #=> "«"     (`MRB_UTF8_STRING` is enabled)

This behavior is incompatible with Ruby, and a little inconvenient because
it can't be interpreted as ASCII-8BIT with `MRB_UTF8_STRING`, I think.

So add encoding argument according to Ruby.

after this patch:

  $ bin/mruby -e 'p 171.chr'                #=> "\xab"
  $ bin/mruby -e 'p 171.chr("ASCII-8BIT")'  #=> "\xab"
  $ bin/mruby -e 'p 171.chr("UTF-8")'       #=> "«"

Allow only `String` for encoding because mruby doesn't have `Encoding`
class, and `"ASCII-8BIT"` (`"BINARY"`) and `"UTF-8"` (only with
`MRB_UTF8_STRING`) are valid value (default is `"ASCII-8BIT"`).

Integrate `Integral#chr` (`Fixnum#chr`) to `mruby-string-ext`

Fix `Module#dup` to frozen module

Set `MRB_STR_ASCII` flag in `String#inspect`

As a side effect, `mrb_time_at()` now takes `mrb_int` instead of
`double` as time arguments.

The Complex class needs `mrb_float` so that it does not work with
`MRB_WITHOUT_FLOAT` anyway.

`String#inspect` can set `MRB_STR_ASCII` flag to receiver and return value
because it checks character byte length.

Now `rand` can be used with `MRB_WITHOUT_FLOAT`; ref #4576

Normal `TYPED_FIXABLE(f,mrb_float)` does not work on 64bit int from
casting problems. The new approach works well, but assumes two's
complement and IEEE-754 floating point numbers.

Because they're defined in both `mruby-string-ext` and `mruby-numeric-ext`
(they seem more natural to define in N, but `mruby-string-ext` depends on
`Integral#chr`).

Before this patch:

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> true

After this patch (same as Ruby):

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> false

Refine `String#chr` test and separate `Fixnum#chr` test

Use stack memory for small name of attr accessors

Fix `String#*` test with `MRB_WITHOUT_FLOAT`

Fix `String#[]` test

Fix `(fixnum_value ... float_value).min` with `mruby-range-ext`

Revert #4300

Allow `mruby-range-ext` to work with `MRB_WITHOUT_FLOAT`; ref 2add8641

Originally, it was not necessary to change. I made mistake.

Before this patch:

  $ bin/mruby -e 'p (2...4.0).min'  #=> TypeError

After this patch (same as CRuby and without `mruby-range-ext`):

  $ bin/mruby -e 'p (2...4.0).min'  #=> 2

Functions that are called infrequently need not to be inline.