Principally, this adds producing libmentat_ffi.a for consumption by Carthage.
To achieve this, we disable bitcode and bump the miniumum iOS version
to 11. In addition, we make things open and public so that consumers
can, well, consume.
`CacheDirection` enum is used only on the Swift side to provide a usable interface. FFI calls are more explicit.
Tests ensure that a cached query is faster than the uncached one.
Documents the FFI layer for Mentat, and provides transaction functionality via an EDN string. Creates two native libraries for iOS (Swift) and Android (Java) and fully tests the FFI for both platforms.
Closes#619#614#611
