Added core and sexplib to oasis dependencies
Use Jane Street's Core as Standard Library

- Use sexp conversions for most types
- Use Core's collection types (Map, Set, Array and Hashtbl)
- Change function calls as needed for the Core equivalents
Added sexplib and comparelib as dependencies
Add sexp and compare to all exposed types
Core: fix function return type warning
Core: Updated travis configuration
core: only depend on core_kernel

Core's huge and has platform-specific dependencies. To keep dependencies
and total compile time down, only depend on the library that this
library uses.
core: remove dependency on comparelib
core: update dependencies in opam and .travis.yml
Merge pull request #53 from frenetic-lang/core

Core