Add a benchmark for measuring call overhead

[alexcrichton]

The goal of this new benchmark, call, is to help us measure the
overhead of both calling into WebAssembly from the host as well as
calling the host from WebAssembly. There's lots of various ways to
measure this so this benchmark is a bit large but should hopefully be
pretty thorough. It's expected that this benchmark will rarely be run in
its entirety but rather only a subset of the benchmarks will be run at
any one given time.

Some metrics measured here are:

• Typed vs Untyped vs Unchecked - testing the cost of both calling wasm
  with these various methods as well as having wasm call the host where
  the host function is defined with these various methods.

• With and without call_hook - helps to measure the overhead of the
  Store::call_hook API.

• Synchronous and Asynchronous - measures the overhead of calling into
  WebAssembly asynchronously (with and without the pooling allocator) in
  addition to defining host APIs in various methods when wasm is called
  asynchronously.

Currently all the numbers are as expected, notably:

• Host calling WebAssembly is ~25ns of overhead
• WebAssembly calling the host is ~3ns of overhead
• "Unchecked" is a bit slower than "typed", and "Untyped" is slower than
  unchecked.
• Asynchronous wasm calling a synchronous host function has ~3ns of
  overhead (nothing more than usual).
• Asynchronous calls are much slower, on the order of 2-3us due to
  madvise.

Lots of other fiddly bits that can be measured here, but this will
hopefully help establish a benchmark through which we can measure in the
future in addition to measuring changes such as #3876