Doug Gregor fb253b182a Use a more efficient SubSequence type for lazy map and filter. ...

Rather than using the default slice type when slicing the collection produced
by a lazy map or filter, slice the base collection and form a new
lazy map/filter collection from it. This allows any optimizations provided by
the collection SubSequence type to kick in, as well as ensuring that slicing
a lazy collection provides the same type as producing a lazy collection of a
slice.

This is technically source-breaking, because someone could have spelled out
the types of slicing a lazy filter or map… but it seems unlikely to matter
in practice and the benefits could be significant.

Fixes ABI FIXME’s #28 and #46.

2017-10-01 15:08:23 -07:00

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