Provide shortcuts for common take idioms

[vic]

Since the following two take idioms are expected to be frequently used (See the warning-aside at https://den.oeiuwq.com/guides/bidirectional/#denprovidesbidirectional)

den.lib.take.exactly({host}: ...)

den.lib.take.atLeast({host, user}: ...)

Why not provide shortcuts for them under den.lib:

den.lib.host aspect

den.lib.user aspect

So that den.lib.host x basically translates to:

den.lib.host = aspect: den.lib.take.exactly({host}@ctx: fixedTo ctx aspect)

den.lib.user = aspect: den.lib.take.exactly({host, user}@ctx: fixedTo ctx aspect)

Not sure about the names, but this is the idea.

[crasm]

I support the idea and suggest perUser and perHost to have an active verbiage.

(like perSystem from flake-parts)

[vic]

yah much better names :) thanks for suggesting them.

[vic]

Implementation and tests at #288