Reorganise hostcalls into submodules

src/hostcalls/mod.rs

@@ -0,0 +1,10 @@
+//! Hostcalls that implement
+//! [WASI](https://github.com/CraneStation/wasmtime-wasi/blob/wasi/docs/WASI-overview.md).
+mod fs;
+mod fs_helpers;
+mod misc;
+mod sock;
+
+pub use self::fs::*;
+pub use self::misc::*;
+pub use self::sock::*;