Start a high-level architecture document for Wasmtime (#3019)

* Start a high-level architecture document for Wasmtime

This commit cleands up some existing documentation by removing a number
of "noop README files" and starting a high-level overview of the
architecture of Wasmtime. I've placed this documentation under the
contributing section of the book since it seems most useful for possible
contributors.

I've surely left some things out in this pass, and am happy to add more!

* Review comments

* More rewording

* typos

crates/runtime/src/instance.rs

@@ -98,21 +98,39 @@ pub trait ResourceLimiter {
     }
 }
 
-/// A WebAssembly instance.
+/// A type that roughly corresponds to a WebAssembly instance, but is also used
+/// for host-defined objects.
 ///
-/// This is repr(C) to ensure that the vmctx field is last.
-#[repr(C)]
+/// This structure is is never allocated directly but is instead managed through
+/// an `InstanceHandle`. This structure ends with a `VMContext` which has a
+/// dynamic size corresponding to the `module` configured within. Memory
+/// management of this structure is always externalized.
+///
+/// Instances here can correspond to actual instantiated modules, but it's also
+/// used ubiquitously for host-defined objects. For example creating a
+/// host-defined memory will have a `module` that looks like it exports a single
+/// memory (and similar for other constructs).
+///
+/// This `Instance` type is used as a ubiquitous representation for WebAssembly
+/// values, whether or not they were created on the host or through a module.
+#[repr(C)] // ensure that the vmctx field is last.
 pub(crate) struct Instance {
     /// The `Module` this `Instance` was instantiated from.
     module: Arc<Module>,
 
-    /// Offsets in the `vmctx` region.
+    /// Offsets in the `vmctx` region, precomputed from the `module` above.
     offsets: VMOffsets,
 
     /// WebAssembly linear memory data.
+    ///
+    /// This is where all runtime information about defined linear memories in
+    /// this module lives.
     memories: PrimaryMap<DefinedMemoryIndex, Memory>,
 
     /// WebAssembly table data.
+    ///
+    /// Like memories, this is only for defined tables in the module and
+    /// contains all of their runtime state.
     tables: PrimaryMap<DefinedTableIndex, Table>,
 
     /// Stores the dropped passive element segments in this instantiation by index.
@@ -124,6 +142,9 @@ pub(crate) struct Instance {
     dropped_data: EntitySet<DataIndex>,
 
     /// Hosts can store arbitrary per-instance information here.
+    ///
+    /// Most of the time from Wasmtime this is `Box::new(())`, a noop
+    /// allocation, but some host-defined objects will store their state here.
     host_state: Box<dyn Any + Send + Sync>,
 
     /// Additional context used by compiled wasm code. This field is last, and