Add core::ops::Bound::from_opt API

[krtab]

This is a proposition for a new API for core::ops::Bound.

Use cases are provided in the doc examples.

Let me know if this needs an ACP/RCP.

[rustbot]

r? @thomcc

rustbot has assigned @thomcc.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[krtab]

We may prefer to have the signature be pub fn from_opt_xxx<U: Into<T>>(opt: Option<U>) -> Bound<T> instead of the current pub fn from_opt_xxx(opt: Option<T>) -> Bound<T>.

[thomcc]

I think this needs an ACP.