Fix class/instance variable name validation

- `@@?` etc are invalid class variable name.
- `@1` etc are invalid instance variable name.

Refine `appveyor.yml`

- Add Visual Studio 2017.
- Enable `shallow_clone` for saving build time.
- Cache extracted WinFlexBison.

Use `FrozenError` instead of `RuntimeError` in `String#rstrip!`

Fix constant name validation

`X!` etc are invalid constant name.

Do not raise an exception when bintest fail

- An exception do not raise when mrbtest fail.
- There are no useful informations in exception message and backtrace.

Fix typo in `AUTHORS` [ci skip]

Reduce `String` creation in `check_(cv|const)_name_sym`

Fix missing assertions in `mruby-string-ext` test

Update `AUTHORS` [ci skip]

Rename `MITL` to `LICENSE` in `.yardopts`; ref 67728c15 [ci skip]

Add bintest header

Add a missing file for #4314

Allow `enable_bintest` without `enable_test` in build config

Set `GEMNAME` on bintest

Refer also CXX and CC env vars as linker command in gcc and clang toolchain

ref #4292

Count skip tests

Extract similar code fragment to method in `src/class.c`

`GEMNAME` is undefined in bintest

ref: https://github.com/mruby/mruby/pull/4296#discussion_r261868710

Simplify `MRubyTestSkip` in `test/assert.rb`