new TASTy header format - add experimental version and tooling version fields

[bishabosha]

Here we adds fields for experimental and tooling versions.

• ExperimentalVersion: Nat, when reading non-zero and
  we are non-zero, the file must be at same version.
  Otherwise if we are non-zero, the file must have
  zero experimental version, and must be of strictly
  lower minor version.
• ToolingVersion: String, an arbitrary UTF-8 string
  that says the name+version of the tooling that
  produced the file. We could reinterpret to mean
  the "minimum" version required to read that file,
  but that would require more process to record that.

fixes #10375

[bishabosha]

This should be reformulated to again swap MinorVersion for (ScalaMajorVersion and ScalaMinorVersion) and include a string field for the compiler version, for error messages, as the compiler could be a fork, or another language.

[bishabosha]

For reviewers, I need to know advice on what should be the contents of the "toolingVersion" field in the header, for example, should we record somewhere the "minimum" compiler version that is compatible with the current TASTy version (and expose it in dotty.tools.dotc.config.Properties), or always pickle the current compiler version.

Also, since #11167 we have been providing the option to return the fields in the header to the user, and we are generating new String objects to store the compiler version, should there be some caching? (note that we do not have access to the Names cache)

[bishabosha]

A fun representation of what is implemented: