lint: add a linter to prohibit map[...]bool in favor of map[...]struct{}

[yuzefovich]

The linter is currently limited to sql/opt/norm package. This commit was prompted by an article mentioned in the Golang Weekly newsletter. The rationale is that map[...]struct{} is more efficient, but in some cases the bool map value is actually desired.

Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

Curious what people think about such a linter in general @cockroachdb/sql-queries.

[michae2]

I'm generally in favor. As you say there are cases where the trinary logic of map[...]bool is actually needed, so it would be nice to have some kind of "nolint" comment or something.

[yuzefovich]

Add an opt-out to disable the linter.

TFTRs!

bors r+

[craig[bot]]

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[craig[bot]]

Build succeeded:

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