It's very common to encounter single-character strings in JavaScript on
the web. We can make such strings significantly lighter by having a
1-character inline capacity on the Vectors.
The current method of iterating through the string to access a code
point hurts performance quite badly for very large strings. The test262
test "RegExp/property-escapes/generated/Any.js" previously took 3 hours
to complete; this one change brings it down to under 10 seconds.
To be used as a RegexStringView variant, Utf16View must provide a couple
more helper methods. It must also not default its assignment operators,
because that implicitly deletes move/copy constructors.
