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/test-programs/tests/wasm_tests/runtime.rs

@@ -49,7 +49,7 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
         .iter()
         .map(|i| {
             let field_name = i.name();
-            if let Some(export) = snapshot1.find_export_by_name(field_name) {
+            if let Some(export) = snapshot1.get_export(field_name) {
                 Ok(export.clone())
             } else {
                 bail!(
@@ -67,7 +67,7 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
     ))?;
 
     let export = instance
-        .find_export_by_name("_start")
+        .get_export("_start")
         .context("expected a _start export")?
         .clone();