[PATCH] Optimize json_lex_string by batching character copying

When parsing JSON strings need to be converted from the JSON string
format to a c-style string. A simple copy of the buffer does not suffice
because of the various escape sequences that that JSON supports. Because
of this our JSON parser wrote characters into the c-style string buffer
one at a time.

However, this is only necessary for these escaped sequences that map to
another character. This patch changes the behaviour for non-escaped
characters. These are now copied in batches instead of one character at
a time.

To test performance of this change I used COPY BINARY from a JSONB table
into another, containing fairly JSONB values of ~15kB. The JSONB values
are a JSON object with a single level. They contain a few small keys and
values, but one very big value that's a stringified JSON blob. So this
JSON blob contains a relatively high number of escape characters, to
escape all the " characters. This change improves performance for
workload this workload on my machine by ~18% (going from 1m24s to 1m09s).

@Andres, there was indeed some low hanging fruit. 
@John Naylor, SSE2 indeed sounds like another nice improvement. I'll leave 
that to you.