Adding type aliases

[konnov]

Per popular request this PR adds type aliases of the form:

@typeAlias FOO = [a: Int, b: Bool];

This closes #703.

• Tests added for any new code
• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Documentation added for any new functionality
• Entry added to UNRELEASED.md for any new functionality

[shonfeder]

Since these patterns overlap, does this mean "is neither a (predefined) typeConst or typeVar"? If so, I'm not clear how the typeVar case applies, since typeVar's aren't defined previously, but just identified syntactically. Should you perhaps constrain aliases to being [A-Z_][a-zA-Z0-9_]* so there's no possibility of confusing a type variable for a type alias?

[shonfeder]

where does PID come from? What does it mean?

[shonfeder]

Suggested change

	We extend the type grammar of `T` with one more rule that introduces a type
	alias:
	We extend the type grammar of `T` with one more rule that introduces type
	aliases:

[shonfeder]

This intro to the section doesn't make sense to me, because the type alias is specified in the grammar to begin with, not added as an extensions here (the way the current document is written). Perhaps just say "kthe grammer of T includes a rule" rather than "we extend"?

[shonfeder]

Is this change related to the feature added in this PR?

[konnov]

Yes. Caching is not consistent with type aliases. If we want to cache parsing results, we have to take the aliases into account. That seems to be too difficult. There is no good use case for caching anyways.

[konnov]

The first solution to aliases was not working well:

• It was introducing a new level of complexity in the type grammar.
• It was sensitive to the order of declarations, which could change with the topological sort (done by the importer).

Here is a new solution. We just let the user redefine a constant type as an alias:

\* @typeAlias: ENTRY = Str;
\* @type: Int -> ENTRY;

I find this approach much more elegant. It also gives us another use for the type constants.

[konnov]

@shonfeder, if you have time, please have a look at it.