[scudo] Make Vector() constexpr

A `Vector` that doesn't require an initial `reserve()` (eg: with a
default, or small enough capacity) can have a constant initializer.

This changes the code in a few places to make that possible:
- mark a few other functions as `constexpr`
- do without any `reinterpret_cast`
- allow to skip `reserve` from `init`

Differential Revision: https://reviews.llvm.org/D107308