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

src/commands/run.rs

@@ -244,7 +244,7 @@ impl RunCommand {
                 let module_name = i.module();
                 if let Some(instance) = module_registry.get(module_name) {
                     let field_name = i.name();
-                    if let Some(export) = instance.find_export_by_name(field_name) {
+                    if let Some(export) = instance.get_export(field_name) {
                         Ok(export.clone())
                     } else {
                         bail!(