Fix broken NaN with `MRB_NAN_BOXING`

Example:

  $ bin/mruby -e '(Float::INFINITY - Float::INFINITY).nan?'
  zsh: segmentation fault

Cause:

  `SET_FLOAT_VALUE` is not used. It is needed for normalizing NaN.

Treatment:

  In my environment, this issue could be reproduced only when
  `infinity - infinity`, however `SET_FLOAT_VALUE` should be used in all
  arithmetic operations (regardless of boxing setting), I think.

  So I fixed all similar codes by extracting to macro.

Refine `assert_float`

The `env` stores stack length in a 10 bit field.
See `MRB_ENV_STACK_LEN()` macro.

Avoid arithmetic operations when `exp` and/or `act` are infinity or NaN.

Remove incorrect flags updating in `mrb_regexp_p()`

Fix buffer overflows in parser.

Use `mrb_immediate_p()` in `mrb_obj_freeze()` and `mrb_obj_frozen()`

Extract frozen checking to function

Remove unneeded function prototypes

Fix test for `Array#slice`

Move `Array#(append|prepend)` from core to `mruby-ary-ext`

Remove unused `mrb_proc_cfunc_p()`

They are not included in ISO standard.

`Module#alias_method` should return `self` in ISO standard

Update version and release date. `mruby 2.0.1 (2019-4-4)`

Use `mrb_proc_arity` instead of `Proc#arity` call in `Method#arity`

Modify `#else` and `#endif` annotations in `src/fmt_fp.c` [ci skip]

`mruby 2.0.1 (2019-4-4)`

Remove unnecessary `_set_output_format` call