strconv: Unquote has custom for-loop-based `contains` implementation

[dgryski]

It should use one of the optimized assembly routines from strings (or I guess the runtime if you don't want to introduce a dependency on strings from strconv).

go/src/strconv/quote.go

Line 421 in c10e614

// contains reports whether the string contains the byte c.

[josharian]

Probably depends on #19792

[gopherbot]

Change https://golang.org/cl/116755 mentions this issue: strconv: use bytealg implementation of IndexByteString