Optimize JSON escaping using SIMD

Here we adjust escape_json_with_len() to make use of SIMD to allow
processing of up to 16-bytes at a time rather than processing a single
byte at a time.  This has been shown to speed up escaping of JSON
strings significantly.

Escaping is required for both JSON string properties and also the
property names themselves, so this should also help improve the speed of
the conversion from JSON into text for JSON objects that have property
names 16 or more bytes long.

Escaping JSON strings was often a significant bottleneck for longer
strings.  With these changes, some benchmarking has shown a query
performing nearly 4 times faster when escaping a JSON object with a 1MB
text property.  Tests with shorter text properties saw smaller but still
significant performance improvements.  For example, a test outputting 1024
JSON strings with a text property length ranging from 1 char to 1024 chars
became around 2 times faster.

Author: David Rowley
Reviewed-by: Melih Mutlu
Discussion: https://postgr.es/m/CAApHDvpLXwMZvbCKcdGfU9XQjGCDm7tFpRdTXuB9PVgpNUYfEQ@mail.gmail.com

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