Document the wasmtime::Instance APIs (#814)

* Document the `wasmtime::Instance` APIs

This documents oddities like the import list and export list and how to
match them all up. Addtionally this largely just expands all the docs
related to `Instance` to get filled out.

This also moves the `set_signal_handler` functions into
platform-specific modules in order to follow Rust idioms about how to
expose platform-specific information. Additionally the methods are
marked `unsafe` because I figure anything having to do with signal
handling is `unsafe` inherently. I don't actually know what these
functions do, so they're currently still undocumented.

* Fix build of python bindings

* Fix some rebase conflicts

crates/api/tests/invoke_func_via_table.rs

@@ -19,7 +19,7 @@ fn test_invoke_func_via_table() -> Result<()> {
     let instance = Instance::new(&module, &[]).context("> Error instantiating module!")?;
 
     let f = instance
-        .find_export_by_name("table")
+        .get_export("table")
         .unwrap()
         .table()
         .unwrap()