syntax: replace if-else chains with expr switch stmt

[quasilyte]

Usually this form is considered more idiomatic.
I've left if-else chains with init statements untouched
to less obvious changes.

Found with elseif check from go-critic linter.

[mvdan]

Please check that the benchmarks don't get affected by this. Especially the parser, as touching the lexer code can lead to noticeable slowdowns.

[quasilyte]

Sure thing.

name     old time/op  new time/op  delta
Parse-8  42.4µs ± 1%  42.2µs ± 1%  -0.57%  (p=0.011 n=9+9)
Print-8  10.3µs ± 0%  10.2µs ± 0%    ~     (p=0.150 n=9+10)

Seems there is hardly any changes at all (-0.57% is a noise here)

[mvdan]

Great, thanks. I presumed so, but you never know.