Add ruby versioning support

[mkruskal-google]

This adds more useable versioning support to rules_ruby.

Downstream projects should call rules_ruby_register_toolchains which will setup the allowed ruby toolchains for each version, including the system installation. The --@rules_ruby//ruby:version=<version> flag can be passed to switch between these dynamically, with the default being whatever is currently installed on the system.

ruby_binary will use the interpreter from whichever toolchain was resolved. Downstream targets can access information about the selected installation via the following targets:

• @rules_ruby//ruby/runtime:headers - The c headers exported by the runtime
• @rules_ruby//ruby/runtime:config_jruby - A setting that specifies that this is a Java implementation of ruby (jruby)
• @rules_ruby//ruby/runtime:config_ruby - A setting that specifies that this is a standard C implementation of ruby

[kigster]

Why did you close this?

[kigster]

In any case there are two separate PRs here:

1. First one deals with the global rename bazelruby_rules_ruby to rules_ruby
2. Second modified the toolchain

Please split them up and resubmit.

[mkruskal-google]

This was never meant to be opened in the first place. I meant to send this out to our fork (https://github.com/protocolbuffers/rules_ruby), where it is two separate PRs. We might try to upstream these changes in the future, but have no immediate plans to