parser: Complete rewrite to LALR (#202)
This replaces the existing Earley parser (which is O(n^3)) with a LALR parser
using Yacc which is an ideal O(n). Even with very short SQL statements, the
existing parser was _really_ slow, so I had to build a query cache as bandaid,
but that has also been removed now.
This refactoring was made possible by adapting yacc from a Go implementation
here: https://github.com/elliotchance/vyac. However, in keeping with the promise
of this repo being completely written in V, the source has been copied to this
repo.
Other notable and breaking changes:
1. Not sure how this worked before, but the query may not specify a catalog in
identity chains (for example, `catalog.schema.table`). The catalog must be set
using `SET CATALOG`.
2. Syntax error messages will be slightly different, but should be a little more
helpful.
3. There are some ambiguities with the SQL grammar, such as trying to decode
what `x IS NOT TRUE` means or differentiating between `COUNT(expr)` vs
`COUNT(*)` due to lookahead limitations. Some special tokens for combinations of
operators and keywords have had to be added for known edge cases, but there are
many remaining conflicts. I suspect these conflicts don't matter as ambiguous
paths should still yield valid results, so these warnings have to be ignored for
now.
4. Fixes a very minor bug where string literals in VALUES might be treated as
`VARCHAR` instead of `CHARACTER` in some cases.
5. Renamed "std_" files with their position number in the standard. This helps
for grouping similar sections and makes lookups easier.
