Refactor away the Instantiator type in Wasmtime

[alexcrichton]

This internal type in Wasmtime was primarily used for the module linking
proposal to handle instantiation of many instances and refactor out the
sync and async parts to minimize duplication. With the removal of the
module linking proposal, however, this type isn't really necessary any
longer. In working to implement the component model proposal I was
looking already to refactor this and I figured it'd be good to land that
ahead of time on main separate of other refactorings.

This commit removes the Instantiator type in the instance module.
The type was already private to Wasmtime so this shouldn't have any
impact on consumers. This allows simplifying various code paths to avoid
another abstraction. The meat of instantiation is moved to
Instance::new_raw which should be reusable for the component model as
well.

One bug is actually fixed in this commit as well where
Linker::instantiate and InstancePre::instantiate failed to check
that async support was disabled on a store. This means that they could
have led to a panic if used with an async store and a start function
called an async import (or an async resource limiter yielded). A few
tests were updated with this.

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[peterhuene]

Just some nits. Looks good!