Move crates/api to crates/wasmtime (#1693)

The `wasmtime` crate currently lives in `crates/api` for historical
reasons, because we once called it `wasmtime-api` crate. This creates a
stumbling block for new contributors.

As discussed on Zulip, rename the directory to `crates/wasmtime`.

crates/fuzzing/Cargo.toml

@@ -15,7 +15,7 @@ log = "0.4.8"
 rayon = "1.2.1"
 wasmparser = "0.52.0"
 wasmprinter = "0.2.1"
-wasmtime = { path = "../api" }
+wasmtime = { path = "../wasmtime" }
 wasmtime-wast = { path = "../wast" }
 
 [dev-dependencies]