Limit worst-case memory for string/depth buffers after simdjson call completes

[TysonAndre]

E.g. if there's a long-running CLI application that calls simdjson_decode() once on startup, for a 100 megabyte long string, then avoid keeping around the 100 megabyte buffer that was allocated for the C simdjson parser after it is finished being used.

This would make it easier to reason about the worst-case impact of simdjson on PHP's memory usage.

If a buffer would be longer than a certain threshold (e.g. 100KB), then use a different short-lived instance (so that the same parser/buffer can be reused when calling simdjson again on small json blobs)