Perform: include filename in Content-Disposition

This changes the meaning of the FileO contents: what used to be
interpreted as the file extension is now used for the whole file name.
rest-stringmap: Initial commit
rest-stringmap: Remove JSONSchema constraint on keys and derive Typeable
rest-types, rest-core: Replace `StringMap [(a,b)]` with rest-stringmap `StringHashMap String a`
Bump rest-types, rest-client, rest-core, rest-gen, rest-snap, rest-happstack