ast: Link new trees to the top of ap->available_current, not the front.
ast: Link new trees to the top of ap->available_current, not the front.

This change is beneficial only in a corner-ish case of a lambda within a
lambda. Prior to this commit, new trees were added to the front of available
start and became the start. This was because, in the past, it didn't make a
difference to do things that way.

Now, it makes a difference, because lambdas will only take trees from the
available current when their body is processed. So if a lambda has two
different lambdas within it, the first may allocate trees at the front that
the second would not be able to see.

This should be unnoticed by normal stuff, but a slight gain to that corner
case.

FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily,jesserayadkins/lily,jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily