This adds a feature which allows one to look up an export by name
without knowing what module it's in -- `lookup_global_export` on an
`InstanceContents`.
The main expected use for this is to support APIs where module A
imports a function from module B, and module B needs to access module
A's memory. B can't import it from A in the normal way, because that
would create a dependency cycle. So for now, allow B to look up A's
exported memory dynamically with `lookup_global_export`.
In the future, with reference types and possibly host bindings, we'll be
able to pass references to memory as arguments, which will obviate the
need for this mechanism.
wasmtime-environ is meant to support cross compilation, so it shouldn't
have dependencies on target layout of structs. This moves the layout
back into wasmtime-execute, and adds a system of asserts for checking
that wasmtime-environ's offsets stay in sync.
The memmap crate doesn't make it straightforward to have part of the
region be writeable and part readonly. Since this is a fairly boutique
use case, and we don't need all that much code, just use the low-level
APIs directly.
Also, introduce a concept of "tunables" for adjusting the parameters of
the runtime.
The biggest change is the split from FunctionIndex to
DefinedFuncIndex to FuncIndex. Take better advantage of this by
converting several Vecs to PrimaryMaps.
Also, table_addr can now handle indices of the table index type,
so we don't need to explicitly uextend them anymore.
