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.

Let inspect recursion do the right thing

Copy receiver's `MRB_STR_ASCII` flag in some methods of `String`

Error needed/conflicts configuration

Unify type of line number to `uint16_t`

Keep `MRB_STR_ASCII` flag in some methods of `String`

Improve `utf8len()` performance with UTF-8

 Fix pack/unpack for base64; ref #4556

For example, `"".unpack("")` evaluates to `[]`.

The pack/unpack "m" directive should be treated as a length rather than
an element count.

The purpose is to clarify the error if there is a needed/conflicts
configuration at compile time.

Also integrated the common parts of `mrb_mod_attr_reader()` and
`mrb_mod_attr_writer()` functions.

Add `mruby-method` gem to `default.gembox` [ci skip]

It's an error in ISO specification; 15.3.2.2.4 and 15.3.2.2.7