Rename the wasmtime_api library to match the containing wasmtime crate (#594)

* Rename the `wasmtime_api` library to match the containing `wasmtime` crate

Commit d9ca508f80 renamed the
`wasmtime-api` crate to `wasmtime`, but left the name of the library it
contains as `wasmtime_api`.

It's fairly unusual for a crate to contain a library with a different
name, and it results in rather confusing error messages for a user; if
you list `wasmtime = "0.7"` in `Cargo.toml`, you can't `use
wasmtime::*`, you have to `use wasmtime_api::*;`.

Rename the `wasmtime_api` library to `wasmtime`.

* Stop renaming wasmtime to api on imports

Various users renamed the crate formerly known as wasmtime_api to api,
and then used api:: prefixes everywhere; change those all to wasmtime::
and drop the renaming.

crates/misc/py/src/memory.rs

@@ -7,11 +7,10 @@ use pyo3::prelude::*;
 use std::ffi::CStr;
 use std::os::raw::{c_int, c_void};
 use std::ptr;
-use wasmtime_api as api;
 
 #[pyclass]
 pub struct Memory {
-    pub memory: api::HostRef<api::Memory>,
+    pub memory: wasmtime::HostRef<wasmtime::Memory>,
 }
 
 #[pymethods]