Array append performance

[dsimcha]

== Quote from superdan (super at dan.org)'s article
> sorry to always be the one who puts the moose on the table. y'all who want to
make capacity a field in the array are off base.
> today each slice has length and data. why. because all a slice is *is* length
and data. it's a view of a portion of memory. the slice don't care where memory
came from. capacity is part of the memory that slices come from. it would be
shared by several slices. and all of a sudden you realize capacity can't be a
field in the slice.

A capacity field would work fine with slicing if, whenever you take a slice into
an array, the capacity field of the slice is set to zero implicitly, even though
the length is longer than zero.  This is in effect how it works now.  Try calling
std.gc.capacity() on a pointer to a slice.  It will always return 0 b/c a slice
doesn't point to the start of a block.  This means that if you try to increase the
size of a slice it will always be reallocated.  Simply setting the capacity field
of slices to 0 when they are created would replicate this in arrays w/ a capacity
field.