Add default instances for LineJoin and LineCap.
Adding offsetTrail and expandTrail.

These use the offestSegment function to build and offset for a whole
trail or expand a trail as though it was stroked with a particular line
style.  Still needed are documentation and an implementation of miter
join.
Adding documentation for offsetTrail and expandTrail.
Add more documentation and miter join.
Merge branch 'master' into expand
Working on offset Loops.
Missed some handling of loops last time.
added arrowAt and arrowBetween
Merging.
Loops now working.  Code could use some refactoring.
Merge branch 'master' into expand
Updated for diagrams-haddock changes.
change Codomain of Located to Point (Codomain a)

switch from

  Codomain (Located a) = Located (Codomain a)

to

  Codomain (Located a) = Point (Codomain a)

When calling atParam on e.g. a Located (Trail R2), I expect to get a
P2, not a Located R2 (though of course the latter can be converted to
the former).

This also means that instead of requiring constraints of the form

  V (Codomain a) ~ V a

we instead have

  Codomain a ~ V a

which makes more sense: we can only add a location to
translation-invariant things, and translation-invariant parametric
things are precisely those things whose codomain is a vector space.
Merge branch 'parametric-Located' of github.com:diagrams/diagrams-lib into expand
Fixed Offset for new instance for Codomain (Located a).
Merge pull request #102 from diagrams/parametric-Located

change Codomain of Located to Point (Codomain a)
Merge pull request #103 from diagrams/expand

Add offsetTrail and expandTrail.
added arrowAt and arrowBetween
export `fullTurn`
new function `angleBetween`