Minor instantiation benchmark updates (#3790)

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 #3787)

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

Cargo.toml

@@ -62,6 +62,7 @@ winapi = { version = "0.3.9", features = ['memoryapi'] }
 memchr = "2.4"
 async-trait = "0.1"
 wat = "1.0.41"
+once_cell = "1.9.0"
 
 [build-dependencies]
 anyhow = "1.0.19"