Add enumerator chain feature (CRuby-2.6 compatible)

range: Refactor range.h/range.c

Add proc composition feature (CRuby-2.6 compatible)

pack: Remove redundant float check in pack_utf8()

Functions order, name, linkage and so on.

The argument is converted to fixnum before calling.

  - Enumerator::Chain
  - Enumerable#chain
  - Enumerable#+

  - Proc#<< and Proc#>>
  - Method#<< and Method#>>

io: Skip TTY test for environments that TTY device is unavailable.

range: Embed edges in RRange on boxing environment.

e.g. GitLab CI

[Breaking changes]

Developers must use following APIs for accessing attributes of RRange
because RRange structure depends on boxing setting.

- mrb_range_beg
- mrb_range_end
- mrb_range_excl_p

Use `__send__` instead of `send` in a Range test.

This assertion is accidentaly passed because `send` was removed from mruby
core so `NoMethodError` is raised and `NoMethodError` is subclass of
`NameError`.

This is also a reason for #4192 as well.

mruby-enumerator: Refine accessors (obj/meth/args/fib).

Fix Yacc running multiple times.

- `fib=` writer is not used.
- All accessors are used as public (e.g. in `initialized_copy`).

For fix Codacy issue.

Example:

  $ MRUBY_CONFIG=<(echo 'MRuby::Build.new{toolchain(:gcc);gem(core:"mruby-bin-mruby");enable_test}') ./minirake

  Before:

    ...
    CC    build/host/mrbgems/gem_init.c -> build/host/mrbgems/gem_init.o
    YACC  mrbgems/mruby-compiler/core/parse.y -> build/host/mrbgems/mruby-compiler/core/y.tab.c
    YACC  mrbgems/mruby-compiler/core/parse.y -> build/host/mrbgems/mruby-compiler/core/y.tab.c
    YACC  mrbgems/mruby-compiler/core/parse.y -> build/host/mrbgems/mruby-compiler/core/y.tab.c
    YACC  mrbgems/mruby-compiler/core/parse.y -> build/host/mrbgems/mruby-compiler/core/y.tab.c
    CC    build/host/mrbgems/mruby-compiler/core/y.tab.c -> build/host/mrbgems/mruby-compiler/core/y.tab.o
    CC    build/host/mrbgems/mruby-compiler/core/y.tab.c -> build/host/mrbgems/mruby-compiler/core/y.tab.o
    CC    build/host/mrbgems/mruby-compiler/core/y.tab.c -> build/host/mrbgems/mruby-compiler/core/y.tab.o
    CC    build/host/mrbgems/mruby-compiler/core/y.tab.c -> build/host/mrbgems/mruby-compiler/core/y.tab.o
    AR    build/host/lib/libmruby_core.a
    ...

  After:

    ...
    CC    build/host/mrbgems/gem_init.c -> build/host/mrbgems/gem_init.o
    YACC  mrbgems/mruby-compiler/core/parse.y -> build/host/mrbgems/mruby-compiler/core/y.tab.c
    CC    build/host/mrbgems/mruby-compiler/core/y.tab.c -> build/host/mrbgems/mruby-compiler/core/y.tab.o
    AR    build/host/lib/libmruby_core.a
    ...

This reverts commit e20d652f.
This change cannot handle hidden dependency between `mruby-print` and
`mruby-sprintf`; ref #4202

Fix `$rake_root_fiber` checking

Simplify full-core.gembox

Dependencies order is auto detected.

https://github.com/mruby/mruby/pull/4101#pullrequestreview-187752352

mruby-bin-mruby: Add test dependency.

`Kernel#p` etc are used.

Refine description for rake test tasks.

Before:
    $ rake -T test
    rake test_test  # run all mruby tests

After:
    $ rake -T test
    rake test  # run all mruby tests

Suppress a struct initializer warning

Suppress missing macro warns

Suppress a compiler (clang) warning bellow:

    src/vm.c:104:38: warning: suggest braces around initialization of
        subobject [-Wmissing-braces]
      const mrb_value mrb_value_zero = { 0 };
                                         ^
                                         {}