`BeginAbsoluteSequence` needs to be optimised

[peppy]

This is a very hot function so we should find a way to implement this without requiring the recursive (allow / performance) overhead. @smoogipoo had an almost-working solution which needs some further thinking (ie. better defining how this method can/can't be used). Opening this issues so we don't forget about it.

[smoogipoo]

I began working on a no-alloc sequences in https://github.com/smoogipoo/osu-framework/tree/no-alloc-sequences-poc . There's still bugs/considerations which we've gone through a bit in Discord, such as whether sequences are a global concept or not, and how should overriding a parent sequence from a nested sequence work.