RUBY-3299 Direct access to mongocrypt_binary_t for better decryption performance

[jamis]

This adds a benchmark for testing the performance of decrypting a document with a large number of encrypted fields (1500), and then changes the implementation so that it accesses the underlying fields of mongocrypt_binary_t directly, rather than indirectly.

The resulting performance improvements are marginal, but consistent:

with indirect mongocrypt_binary_t access:
  thread_count=1; median ops/sec=44
  thread_count=2; median ops/sec=48
  thread_count=8; median ops/sec=78
  thread_count=64; median ops/sec=327

with direct mongocrypt_binary_t access:
  thread_count=1; median ops/sec=47
  thread_count=2; median ops/sec=52
  thread_count=8; median ops/sec=82
  thread_count=64; median ops/sec=360