COND statement added

[Brian Rice]

I've added conditions: and conditions:otherwise: to complement  
caseOf: and caseOf:otherwise: to src/lib/sequence.slate. Basically  
this is a Lisp-style COND statement, where:

conditions:
   {[testBlock] -> [actionBlock].
    ...} otherwise: [failOverBlock].

Will run the testBlock's in turn, and execute the first actionBlock  
associated with a True-result. It'll return the result of that  
actionBlock and not evaluate further. If no testBlock returns True,  
the failOverBlock is run and its result returned.

There is no byte-compiler optimization for this, but there  
conceivably could be, as long as all the blocks are given literally  
(i.e. in brackets). There's a sketchy version of this that will be  
uploaded shortly but it has to do extra work whereas caseOf: has a  
special bytecode to quickly check lots of literals.

Note of course that Slate's dispatch system and the use of dynamic  
mixins in conjunction can do via polymorphism most of what this idiom  
can do, so it really shouldn't show up much. However, it'd help in  
identifying places where better dispatch algorithms (pattern-matching  
or predicate dispatch) would specifically help.

--
-Brian
http://tunes.org/~water/brice.vcf

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