Ensure nom-sql Display round-trips with parsing for select statements

[glittershark]

Write a proptest to ensure that for all select statements, converting them to a string (using either dialect) and then re-parsing results in the same statement. This is important once we're storing stringified select statements in the authority to use for backwards compatibility.

[lukoktonos]

Will pick up this one, https://gerrit.readyset.name/c/readyset/+/5986 is the starting point

[lukoktonos]

Made some fixes, still more to uncover for full query coverage.

Will come back to this after getting the basic functionality implemented.

https://gerrit.readyset.name/c/readyset/+/6044 nom-sql: Support parsing floats with no RHS

https://gerrit.readyset.name/c/readyset/+/6045 nom-sql: Parse integer literals as signed by default

https://gerrit.readyset.name/c/readyset/+/6046 nom-sql: Fix Variable parsing/display for postgres

[lukoktonos]

had to revert https://gerrit.readyset.name/c/readyset/+/6046 since SqlIdentifier Arbitrary is able to generate keywords in that state which results in invalid Variables being generated. Need to do some filtering to prevent that.

[lukoktonos]

https://gerrit.readyset.name/c/readyset/+/6274