Reduce how long it takes to walk the varrefs in an expression

[jsternberg]

This is used quite a bit to determine which fields are needed in a
condition. When the condition gets large, the memory usage begins to
slow it down considerably and it doesn't take care of duplicates.

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

[jsternberg]

Benchmark comparison:

benchmark                old ns/op     new ns/op     delta
BenchmarkExprNames-8     131087        8389          -93.60%

benchmark                old allocs     new allocs     delta
BenchmarkExprNames-8     308            10             -96.75%

benchmark                old bytes     new bytes     delta
BenchmarkExprNames-8     135424        240           -99.82%

[e-dard]

LGTM 👍

In my testing, this improved a SELECT ... WHERE (lots of ORs) ORDER BY TIME DESC LIMIT 1 query by around 57%.