Enabled the vec![] macro to use the [a; b] repeat syntax.

[Kimundi]

Closes #15587

[rust-highfive]

r? @aturon

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[alexcrichton]

I'm ok with this, but @aturon had worries about vec! on #20657, so I'd like to run this by him.

[aturon]

My worries were just about stabilizing it at all (given that we might want something more general). But that seems unlikely to land soon enough, and we should probably avoid breaking code using vec. I'm fine with this change.

[aturon]

Note that this is a nice replacement for from_elem.

[Kimundi]

Yeah, the from_elem use case is what reminded me about this. Though vec![a; b] is more restrictive since it demands a: Copy and constexpr b, but that should still cover a lot.

[nikomatsakis]

Nifty.