Order of *CENTRAL-REGISTRY* traversal

(This is parallel to my earlier note about 
SB-EXT:*MODULE-PROVIDER-FUNCTIONS* traversal.)

Currently ASDF traverses *CENTRAL-REGISTRY* head-to-tail: new locations 
pushed to the front take precendence and older ones serve as fallbacks. 
The behaviour undocumented.

I believe that this is an unfortunate design as it means that a seemingly
innocent PUSH can lead to totally different set of libraries being loaded
as would be without. The system is unstable: a single idiomatic operation
can have large and impossible to predict consequences.

I would like to change ASDF to traverse it tail to head instead, so that 
(PUSH <DIR> *CENTRAL-REGISTRY*) can only make a previously failing (OOS 
'LOAD-OP :FOO) succeed, but not alter the behaviour of a succeeding 
operation.

This _would_ cause migration work for people whose setups deliberately 
operate on the head or tail already. However, I doubt this is much 
depended on outside personal initialization files since the current 
behaviour is not only undocumented but unstable.

Cheers,

  -- Nikodemus              Schemer: "Buddha is small, clean, and serious."
                   Lispnik: "Buddha is big, has hairy armpits, and laughs."


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