Use Vitess's fork of goyacc, which uses more efficient token union type

[jhump]

This is just a draft because I don't want to actually merge in its current form.

We'd need to make a decision about whether to use --fast-append mode (an optimization intended to counter the fact that the use of an interface type means that non-pointer types are still allocated on the heap, including slices where the slice headers get allocated on the heap) or to wrap all of the slice types in the union in single-field structs (so the value in the interface is a pointer, and there won't be other unintentional allocations of non-pointer types).

If we go with the former, the "unsafe" import is required. But that seems not-great for portability. However, the second approach doesn't immediately let us remove the "unsafe" import until this bug is fixed.

[jhump]

cc @pkwarren

[jhump]

A new experimental parser is being implemented, and it is hand-written: it will not use yacc. So this change is no longer relevant to the direction of this repo.