feat(spanner/spansql): case insensitive parsing of keywords and functions

[dsymonds]

This handles the most important parts of the parser as it relates to
case insensitivity, namely permitting the lexical tokens for the grammar
(as opposed to elements like table/column names) to have arbitrary case.

This includes a complete audit of the parser to check all the lexical
token handling, and cleaned up a few places that were particularly
ancient.

The rendered output remains canonically uppercase.

Fixes #4032.

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[dsymonds]

@googlebot I signed it!

[dsymonds]

@olavloite

[hengfengli]

LGTM.

[hengfengli]

@dsymonds Can you please rebase this branch? Thanks.

[dsymonds]

Rebased!

[hengfengli]

Missed the chance to merge again, :-(. I will try to rebase it if I can.

[hengfengli]

I don't see a button on this PR to allow me to rebase it. Also, I tried to bring the PR to local and rebase it. But when I push it, it gets rejected. @dsymonds can you please rebase it again? sorry for the trouble.

[dsymonds]

rebased, @hengfengli