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

Three-digit exponent issue was fixed via another workaround (63b8f5cf).

Pad leading zero to month and day in `MRUBY_RELEASE_DATE`

For Ruby compatibility.

Fix modifiable class name

Fix the following example:

    Object.const_set :A, Module.new{const_set :B, Class.new}
    ab = A::B.to_s
    p ab         #=> "A::B" # Good
    ab[0] = "x"
    p A::B.to_s  #=> "x::B" # Bad

The addresses for packed inline symbols reference `mrb->symbuf` that
could be overridden by the later call of `mrb_sym2name_len`. Since
file names in call stack information are kept as symbols, keeping the
address in the C structures could cause problems like #4342.

This changes small incompatible changes in function prototypes:
* `mrb_parser_get_filename`: return value changed to `mrb_sym`.
* `mrb_debug_get_filename`: add `mrb_state*` as a first argument.
* `mrb_debug_get_line`: ditto.

I believe above functions are almost internal, and no third-party
mrbgem use them.

Fix warning: '*' interpreted as argument prefix

Add `pass` and `flunk` to `test/assert.rb`