Replace static bufs with a StringInfo in cash_words()

Replace static bufs with a StringInfo in cash_words()

For clarity. The code was correct, and the buffer was large enough,
but string manipulation with no bounds checking is scary.

This incurs an extra palloc+pfree to every call, but in quick
performance testing, it doesn't seem to be significant.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/7f86e06a-98c5-4ce3-8ec9-3885c8de0358@iki.fi