Reorganize host.rs and wasm32.rs. (#151)

* Reorganize host.rs and wasm32.rs.

Reorganize host.rs and wasm32.rs into host.rs, wasi.rs, and wasi32.rs.

Most of the contents of host.rs was not actually host-specific, as most
of the types are fixed-size types like u32 or i64. These types are now
in wasi.rs.

The few types which do have pointer or usize-sized values now remain,
in two versions: host.rs has versions which use actual raw pointers and
usize, and wasi32.rs has versions which use u32 to represent them.

* Fix compilation on BSD

* Fix compilation on Windows

* Fully encapsulate endianness in memory.rs.

This refactors memory.rs to fully encapsulte endianness concerns, so
that outside that file, all values are in host-endian order.

This adds a dependency on the `num` crate, though it's only used for
the `PrimInt` trait, for handling endianness in a generic way.

* Use pub(crate).

Cargo.toml

@@ -28,6 +28,7 @@ cfg-if = "0.1.9"
 log = "0.4"
 filetime = "0.2.7"
 lazy_static = "1.4.0"
+num = { version = "0.2.0", default-features = false }
 
 [target.'cfg(unix)'.dependencies]
 nix = "0.15"