runewidth.StringWidth is an expensive call, even if the input string is pure
ASCII. Improve this by providing a wrapper that short-circuits the call to len
if the input is ASCII.
Benchmark results show that for non-ASCII strings it makes no noticable
difference, but for ASCII strings it provides a more than 200x speedup.
BenchmarkStringWidthAsciiOriginal-10 718135 1637 ns/op
BenchmarkStringWidthAsciiOptimized-10 159197538 7.545 ns/op
BenchmarkStringWidthNonAsciiOriginal-10 486290 2391 ns/op
BenchmarkStringWidthNonAsciiOptimized-10 502286 2383 ns/op
26132cf5bd