Make allocate @safe

[edi33416]

For allocators that implement their own allocate method, this should be a pure nothrow @safe function. Allocators that are building on top of such allocators should infer the function attributes from their parents.

This PR is a subset of #5330, as this approach will provide us with better granularity. More smaller PRs to come :)

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[andralex]

@edi33416 I suggest for now let's leave AffixAllocator with an unsafe allocate.

[wilzbach]

@edi33416 I recommend splitting this PR up into smaller chunks, s.t. it can be reviewed and merged more quickly. As an additional benefit your chances of running into merge conflicts decrease too ;-)

[andralex]

Per the convo with @edi33416 today, please convert this diff to making allocate only nothrow and (where applicable) @nogc.

[wilzbach]

Ping @edi33416

[RazvanN7]

@edi33416 Any chance of finishing this?