Re-merge exec-wasmtime crates once Cargo supports multidep

[bstrie]

In #1794 we split the exec-wasmtime crate out into separate library and binary crates. The motivation for this was to keep CI times from increasing; while the exec-wasmtime binary is a mandatory binary dependency (built for the linux-musl target), the exec-wasmtime library (built for the host's target) can be optional, which leads to noticeable build time wins since we can potentially avoid building the wasmtime crate twice. The long-term fix for this is a new Cargo feature called "multidep": rust-lang/cargo#10061 , which is an extension of binary dependency support ( #1445 ), which allows duplicate dependency declarations when binary dependencies are involved. Once that support lands, we can merge the exec's library and binary crates back together, and depend on it once as a normal optional dependency and once as a mandatory binary dependency with no problems.

[bstrie]

Blocked on #1799 .