Restore GC arena frequently

Remove unnecessary assignments

Add an annotation about the return value

The return value from `mrb_delete_key` needs to be protected from GC in
some cases.

Remove unused node type in `codegen()`

Simplify arguments check in `String#rindex`

Also fix document about type of the first argument.

Use type predicate macros instead of `mrb_type` if possible

For efficiency with `MRB_WORD_BOXING` (implement type predicate macros for
all `enum mrb_vtype`).

The following two may be different:
* `%d` for `int`
* `%i` for `mrb_int`

Use inttypes for `snprintf()`

Keep the type of `posarg` consistent

Remove `MRB_TT_HAS_BASIC` macro

Fix `MRB_WITHOUT_FLOAT` reversal; fix #4598

Add "fall through"

* mrb_sym2name -> mrb_sym_name
* mrb_sym2name_len -> mrb_sym_name_len
* mrb_sym2str -> mrb_sym_str

Match the type with the caller and related functions.

The value of `MRB_TT_HAS_BASIC` is meaningless because `MRB_TT_HAS_BASIC` is
no longer used with `MRB_WORD_BOXING` at b2c3d88f.

`Exception#initialize` should not allow two or more arguments

Fix `Fixnum` overflow test in `Integer#<<` test

- Skip when `MRB_WITHOUT_FLOAT` is defined.
- Make `Fixnum` overflow even when `MRB_INT64` is defined.

Implement all type predicate macros for `MRB_WORD_BOXING`

The default implementations of type predicate macros use `mrb_type`.
But `mrb_type` with `MRB_WORD_BOXING` isn't very efficient, so the new
implementations avoid `mrb_type`.

Rename MRB_USE_ETEXT_EDATA to MRB_USE_LINK_TIME_RO_DATA_P and support lld linked programs

In lld linked programs, .rodata comes before .text, thus mrb_ro_data_p
will return false for strings in .rodata. Change the lower bound from
_etext to __ehdr_start to catch these cases. This works for ld.bfd, gold
and lld, and it does not have false positives even if .init_array does
not exist.

Remove the branch that uses _edata: strings in .data can be modified so
this is semantically incorrect. Delete the __APPLE__ branch (its
manpages say get_etext() and get_edata() are strongly discouraged).
.init_array has been adopted by most ELF platforms to supersede .ctors.

Neither _etext nor _edata is used, so rename MRB_USE_ETEXT_EDATA to
MRB_USE_EHDR_START.

Use `mrb_define_method` instead of `mrb_define_alias`

Singleton class definition do not introduce its own class variable scope
in CRuby/JRuby. So should mruby.

```
module Mod1
  class << Object.new

    C = 1
    @@cv = 1

    p Module.nesting,        # => [#<Class:#<Object:0x55cb16e60a50>>, Mod1]
      constants,             # => [:C]
      class_variables,       # => []
      Mod1.class_variables   # => [:@@cv]
  end
end
```