Fixes to OneOf, add optional, better errors.

5fea5dc|| Add error context messages (#11).
bdc8302|| `optional` combinator. Resolves #18.
640aaa0|| Add scala doc to Primitives for #9.
35fa39f|| Implement JVM UInt/ULong newtypes. Fixes #1, #12.
6fc62c7|| Squashed commit: working OneOf; fixes #16, see #7.
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