Minor instantiation benchmark updates

[alexcrichton]

This commit has a few minor updates and some improvements to the
instantiation benchmark harness:

• A once_cell::unsync::Lazy type is now used to guard creation of
  modules/engines/etc. This enables running singular benchmarks to be
  much faster since the benchmark no longer compiles all other
  benchmarks that are filtered out. Unfortunately I couldn't find a way
  in criterion to test whether a BenchmarkId is filtered out or not so
  we rely on the runtime laziness to initialize on the first run for
  benchmarks that do so.

• All files located in benches/instantiation are now loaded for
  benchmarking instead of a hardcoded list. This makes it a bit easier
  to throw files into the directory and have them benchmarked instead of
  having to recompile when working with new files.

• Finally a module deserialization benchmark was added to measure the
  time it takes to deserialize a precompiled module from disk (inspired
  by discussion on Use mmap'd *.cwasm as a source for memory initialization images #3787)

While I was at it I also upped some limits to be able to instantiate
cfallin's spidermonkey.wasm.