introduce int cells to avoid object allocation
intcells are also closure cells, but with possibly only an intvalue
nits
Merge remote-tracking branch 'origin/master' into tim/intcells
update test
Implement Array#assoc and #rassoc
Untag assoc_spec and rassoc_spec
Implement Integer#downto.
encourage people to untag rubyspecs rather than write their own tests, if possible
Merge remote-tracking branch 'origin/master' into tim/intcells
Raise NotImplementedError for Integer#downto with no block.
Merge pull request #387 from jerith/jerith/integer-downto

Implement Integer#downto.
fix trace
Remove assoc/rassoc tags
Add String#insert
Assert that index in String.insert is non-negative
Implement Array#slice and Array#slice!
String.insert: fix the error message and indentation
Use space.subscript_access() better.
Add File.unlink