It's now no longer necessary to have the versions taking a Type{T}
parameter, since the unary version of map now correctly inferrs the
output FSA type.  (And the comment here was wrong.)

* Rename similar to similar_type since it produces slightly different
  output.
* Implement default similar_type which returns the type unchanged if it
  exactly matches, and falls back to Vec/Mat otherwise
* Implement similar_type for Vec,Point,Mat and tests as necessary.

* Document FSA abstract types which are actually in use (no docs for
  Mutable FSAs yet, since I'm not sure anybody is using them)
* Document concrete FSA types provided by the package
* Document the requirement to overload `similar_type` in certian cases.

* Avoid some duplication in similar_type definitions
* Move `@pure` somewhere more sensible
* Remove similar_type convenience function for simplicity

Make similar_type() work by default in nearly all circumstances, by
introspecting the type tree using fsa_abstract(), supertype() and a
bunch of other ugly stuff involving `TypeVar`s.  Ugh... just ugh, but I
hope it'll make it easier for users.

* Remove the specialized similar_type() implementations for concrete
  types as they're no longer needed
* Fix up README to reflect the new implementation
* Add some extra tests