[scudo] Rework Vector/String

Some platforms (eg: Trusty) are extremelly memory constrained, which
doesn't necessarily work well with some of Scudo's current assumptions.

`Vector` by default (and as such `String` and `ScopedString`) maps a
page, which is a bit of a waste. This CL changes `Vector` to use a
buffer local to the class first, then potentially map more memory if
needed (`ScopedString` currently are all stack based so it would be
stack data). We also want to allow a platform to prevent any dynamic
resizing, so I added a `CanGrow` templated parameter that for now is
always `true` but would be set to `false` on Trusty.

Differential Revision: https://reviews.llvm.org/D103641