wasm: isolate dyld side effects that might require IPC

This commit spins out a DyLDHost class from DyLD that handles side
effects that must be run in the same host environment that runs
wasm32-wasi-ghc. When the dyld script runs in the browser, it'll need
to do IPC to find libraries, fetch wasm library, etc, and the other
side of dyld that runs on nodejs would simply expose the DyLDHost
methods as endpoints for WebSockets/HTTP.

(cherry picked from commit 22ba2a78)