Also does `if` support, discussed refactorings, like PopState/PushState instead of edits.
With these changes, I'm to where I can start talking about what's still missing:
```
//toolchain/parser/testdata:basics/fail_invalid_designators.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/fail_paren_match_regression.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:basics/function_call.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/package.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/structs.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/tuples.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:basics/var.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/fail_colon_instead_of_in.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:for/fail_missing_in.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:for/fail_missing_var.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/nested.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:for/simple.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:function/definition/with_params.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:operators/associative.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:operators/fail_missing_precedence_and_or.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:operators/fail_missing_precedence_or_and.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:operators/fail_variety.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:operators/fixity.carbon.test FAILED in 0.9s
//toolchain/parser/testdata:operators/missing_precedence_not.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:operators/postfix_unary.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:operators/prefix_unary.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:while/basic.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:while/fail_unbraced.carbon.test FAILED in 0.6s
```
This does modify a couple `if` tests to not test so much expression syntax -- that just seems like unrelated syntax.
