Proposal: Support grammar aliases.

In moderately complex grammars it's fairly common to see duplicate patterns emerge. For example, when matching a dot-separated identifier (eg. `foo.bar`) the pattern `(Ident { "." Ident })` is used repeatedly. This can be handled by a Go type alias implementing the `Parseable` interface, but that is quite onerous.

I propose adding support for grammar aliases. Here's an example creating and using an `Identifier` alias:

```go
type Assignment struct {
Name string `@Identifier=(Ident { "." Ident })`
Variable string `"=" @Identifier`
}
```