Swap self and other in vec::append when it avoids a reallocation#77538

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the8472 wants to merge 2 commits intorust-lang:masterfrom

the8472:swap-append

Commits on Oct 4, 2020

vec::append: swap vecs instead of moving elements when the LHS is empty
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reword guarantee section to leave wiggle room for non-(de)allocating shrinks
the8472
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