Fixes C++03 compilation.

Binding

e.g. `def ==(v) = true`. The issue is reported by @shuujii

- prohibit defining setter method
- allow endless def without parentheses

Bump actions/setup-node from v2.1.4 to v2.1.5

Bumps [actions/setup-node](https://github.com/actions/setup-node) from v2.1.4 to v2.1.5.
- [Release notes](https://github.com/actions/setup-node/releases)
- [Commits](https://github.com/actions/setup-node/compare/v2.1.4...46071b5c7a2e0c34e49c3cb8a0e792e86e18d5ea)
Signed-off-by: dependabot[bot] <support@github.com>

- Added to `mruby-binding-core`
  - `Binding#local_variable_defined?`
  - `Binding#local_variable_get`
  - `Binding#local_variable_set`
  - `Binding#local_variables`
  - `Binding#receiver`
  - `Binding#source_location`
  - `Binding#inspect`
- Added to `mruby-proc-binding`
  - `Proc#binding`

The reason for separating `Proc#binding` is that core-mrbgems has a method that returns a closure object to minimize possible problems with being able to manipulate internal variables.
By separating it as different mrbgem, each user can judge this problem and incorporate it arbitrarily.

Make changes to make `Binding` work.
At the same time, it separates `Binding#eval`, which depends on `mruby-eval`, from `mruby-binding-core`.

Updated `doc/opcode.md` to match `include/mruby/ops.h` [ci skip]

Comment out `warn` used in the `Array#fetch` method

I get an error because the current mruby does not have a `Kernel#warn` method.
But the warning itself is useful and I'll just comment it out in case it's implemented in the future.

The issue is reported by @shuujii.

Fix type tag that set to `NilClass` and `FalseClass`

Skip --depth=1 when it's locked

Add `-s` option to `mrbc` for make variable static

Bump actions/upload-artifact from v1 to v2.2.2

Bump actions/cache from v2 to v2.1.4

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from v1 to v2.2.2.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v1...e448a9b857ee2131e752b06002bf0e093c65e571)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [actions/cache](https://github.com/actions/cache) from v2 to v2.1.4.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2...26968a09c0ea4f3e233fdddbafd1166051a095f6)
Signed-off-by: dependabot[bot] <support@github.com>

feat: add Dependabot config file

For better coverage.

Refine checking for trailing whitespace [skip travis][skip appveyor]

Remove trailing tab in `build_config/dreamcast_shelf.rb` [ci skip]

Introduce EditorConfig [skip ci]

Place optable in ROM