Refactor the compilation and instantiation pipeline.

wasmtime-execute is now wasmtime-jit. Move `JITCode` and the TargetIsa
into a new `Compiler` type. `InstancePlus` is no more, with trampoline
functionality now handled by `Compiler`.

lib/jit/src/target_tunables.rs

@@ -0,0 +1,22 @@
+use std::cmp::min;
+use target_lexicon::{OperatingSystem, Triple};
+use wasmtime_environ::Tunables;
+
+/// Return a `Tunables` instance tuned for the given target platform.
+pub fn target_tunables(triple: &Triple) -> Tunables {
+    let mut result = Tunables::default();
+
+    match triple.operating_system {
+        OperatingSystem::Windows => {
+            // For now, use a smaller footprint on Windows so that we don't
+            // don't outstrip the paging file.
+            // TODO: Make this configurable.
+            result.static_memory_bound = min(result.static_memory_bound, 0x100);
+            result.static_memory_offset_guard_size =
+                min(result.static_memory_offset_guard_size, 0x10000);
+        }
+        _ => {}
+    }
+
+    result
+}