Extract token stream interface from parser

[jsatdapr]

Issue To Be Solved

The parser-guided fuzzer needs to have a couple alternate implementations of token buffering and token access, and it needs to be switchable at time of Parse(). Previously I tried to use a buildtag to isolate the different implementations, but the parser-guided fuzzer needs to switch at runtime, and needs more than 2 implementations which makes the buildtag approach even more difficult.

Tradeoff

• There could be a performance hit due to the indirection, but existing prototype work shows no noticeable change in the running time for the full cadence test suite.

Context

The parser-guided-fuzzer is blocked on this.

[jsatdapr]

PR coming shortly