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Follow-up to elastic#126492 to apply the json parsing optimization to strings
containing unicode characters and some backslash-escaped characters.

Supporting backslash-escaped strings is tricky as it requires modifying the
string. There are two types of modification: some just remove the backslash
(e.g. \", \\), and some replace the whole escape sequence with a new
character (e.g. \n, \r, \u00e5). In this implementation, the optimization
only supports the first case--removing the backslash. This is done by
making a copy of the data, skipping the backslash. It should still be more
optimized than full String decoding, but it won't be as fast as 
non-backslashed strings where we can directly reference the input bytes.

Relates to elastic#129072.