Defer several build libraries loading until needed

Fix `Range#max` test (`TypeError` is raised) on 32-bit mode

Suppress warning by gcc with `-Wmaybe-uninitialized`

Create a symbolic link in the temporary directory; fix #4642

Remove unused constant in `Rakefile`; ref e312842a

Because the linker gives a warning on FreeBSD 12.0.

```
warning: mktemp() possibly used unsafely; consider using mkstemp()
```

Merge pull request #4662 from shuujii/fix-RBreak-exceeding-6-words-on-32-bit-mode-w-o-boxing-and-MRB_USE_FLOAT

Fix `RBreak` exceeding 6 words on 32-bit mode w/o boxing and `MRB_USE_FLOAT`

ref: https://github.com/mruby/mruby/pull/4483#issuecomment-498001736

In this configuration, `tt` of `RBreak::val` is set into `RBreak::flags`.

Refine processing for gem lock file

- Defer YAML library and lock file loading until needed.
- Don't write empty parts into lock file.
- Extract code to read/write lock file to `MRuby::Lockfile`.
- `MRuby::Lockfile.disable` disables the use of lock file.

make clean error workaround

Take commit hash of mruby too

Separate repos directory and build directory

closes #4652

Support lock file for git.

Prioritize embedded string over nofree (or normal) string

Prioritize embedded string in the following functions:

- `str_new_static`
- `str_new`
- `mrb_str_new_capa`
- `mrb_str_pool`

The reasons are as follows:

- Consistency with `mrb_str_byte_subseq` and `str_replace`.
- Memory locality increases and may be slightly faster.
- No conversion cost to embedded string when modifying the string.

Remove deprecated iij stuffs

Fix link of mruby-io

Use mruby-onig-regexp instead in example

Add note about checksum_hash in mrbgem doc

Based on code proposed by @dearblue to avoid the warning:
`dereferencing type-punned pointer will break strict-aliasing rules`.

Use `RBasic` padding for embedded string on 64-bit CPU

On 64-bit CPU, there is padding in `RBasic`, so reorder the fields and use
it as buffer of embedded string. This change allows 4 more bytes to be
embedded on 64-bit CPU.

However, an incompatibility will occur if `RString::as::ary` is accessed
directly because `RString` structure has changed.

Move `mrb_str_pool` to `src/string.c` to use `str_init` family

Allow external definition of MRB_API macro

When building WebAssembly targets with clang (e.g. wasm32-unknown-unknown), all
symbols are hidden by default. To export a symbol, it must be marked with

    __attribute__((visibility(default)))

Rather than detecting this exotic target in mruby, allow MRB_API to be defined
externally.