mruby-time: support time_t is uint

It was default on Linux. Abandoned for 2 reasons:

* some cross-platform compiler defines `__linux__` even on bare metal
  environment (RTL8196 (MIPS big endian soc) for example).

* some compilers do not align functions pointers so that we need to
  specify `-falign-functions=n` (where n>1). It's not suitable for
  default configuration.

By our measurement, `mrbtest` consumes 400KB less memory. So if your
target machine is memory tight, add the following command-line options
to `GCC` (unfortunately `clang` does not support `-falign-functions`).

`-falign-functions=2 -DMRB_METHOD_TABLE_INLINE`

Should be true for close_on_exec flag

When your target machine is big endian, specify as following in the
`build_config.rb`:

```ruby
MRuby::CrossBuild.new('32bit') do |conf|
  toolchain :gcc

  conf.big_endian
end
```

On some platform and timezone it is a valid time spec.

mruby-time: Fix mruby specific timegm() cannot return minus

Set the mruby-1.4.0 release date to `2018-1-16`.

Passed mruby-test on Cygwin.

In mruby, those methods are defined in `Integral` module.

Since mruby mixes `Integer` and `Float`, integer operations have
been moved to `Integral` module.

To distinguish string keys and symbol keys.

Also removed tests that depends on implementation details of the default
`method_missing` behavior.

This information is not mandatory but causes a lot of problems in the
past due to infinite recursion by redefining `to_str`, `inspect` etc.

The receiver can be switched using `#instance_eval` etc.

For example, the following code behaved wrong:

```ruby
Object.instance_exec(1,2,3){|*a|
  def foo
    42
  end
}
p foo()
```

Make source compilable with C++17

Changes applied:

- Removing "register" keyword
- Fixing const pointer to pointer assignments
- Adding type casts to rb_malloc calls