Allow functional expansion

[tfausak]

It should be possible to provide a function, or perhaps a monadic action, to replace variables with their values. That way you could look them up from the environment, read them from the database, or have them depend on the current time.

There is one small wrinkle here from section 3.2.1:

If a variable appears more than once in an expression or within multiple expressions of a URI Template, the value of that variable MUST remain static throughout the expansion process (i.e., the
variable must have the same value for the purpose of calculating each expansion). However, if reserved characters or pct-encoded triplets occur in the value, they will be pct-encoded by some expression types and not by others.

So when replacing in this manner, each value must be requested at most once.

[tfausak]

The expandTemplate function already kind of allows this:

burrito/src/lib/Burrito/Expand.hs

Lines 39 to 43 in 41ea35a

	expandTemplate
	:: Applicative m
	=> (Name.Name -> m (Maybe Value.Value))
	-> Template.Template
	-> m String

I think the constraint would have to change to Monad m in order to cache lookups, but otherwise that should work nicely.