Add option::get_zero

[catamorphism]

r? @brson - as per #3797

[brson]

r+

[catamorphism]

Merged, 1f1e7e9

[kud1ing]

get_zero is in line with get_default. But to me it sounds a bit like "give zero".

How about get_or_zero and get_or_default?

[nikomatsakis]

I personally'd prefer @kud1ing's suggestions. The name get_default() always throws me when I read it.

[erickt]

+1 to @kud1ing's naming scheme.

[kud1ing]

I often think, that variable/function names should not be less descriptive than their short documentation string. And in this case it is "Returns the contained value or zero".

That's why i'd even prefer value_or_zero() and value_or_default() but maybe that would be considered too long?

[nikomatsakis]

I think they should use the same prefix as get(). And I prefer opt.get() to opt.value(). It's shorter but also using a verb conveys the fallibility to me. value() sounds like it's just a field getter.

[catamorphism]

Seems like the masses have spoken in favor of get_or_default and get_or_zero, so I checked in 89acd1f