Gate support for the wasm threads proposal behind a Cargo feature

[alexcrichton]

This commit moves support for the threads proposal behind a new on-by-default Cargo feature: threads. This is intended to support building Wasmtime with fewer runtime dependencies such as those required for the atomic operations on memories.

This additionally adds the gc feature in a few missing places too.

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[abrown]

Looks good to me; I assumed all the shared memory and parking spot changes were just refactoring, no functional changes.