Rewrite linear memory handling in terms of simple mmap/VirtualAlloc.

The memmap crate doesn't make it straightforward to have part of the
region be writeable and part readonly. Since this is a fairly boutique
use case, and we don't need all that much code, just use the low-level
APIs directly.

Also, introduce a concept of "tunables" for adjusting the parameters of
the runtime.

lib/execute/src/lib.rs

@@ -30,7 +30,7 @@
 extern crate cranelift_codegen;
 extern crate cranelift_entity;
 extern crate cranelift_wasm;
-extern crate memmap;
+extern crate errno;
 extern crate region;
 extern crate wasmtime_environ;
 #[cfg(not(feature = "std"))]