core: no-std panic::AssertUnwindSafe (etc.)

[brodycj]

STATUS:

I think the following PRs should be merged first to resolve the CI build issues:

• resolve unnecessary map_or Clippy issue #2904 - resolve clippy failure
• Bump general MSRV to 1.63 #2907 - bump main / general MSRV to resolve MSRV failure
• Bump MSRV of core crates to 1.56.0 #2906 - bump core MSRV as needed for this PR

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WHAT:

• use panic::AssertUnwindSafe from Rust core lib in futures-core
• remove extern crate std not needed from futures-core & futures-sink

I would be happy to split these updates into separate PRs, if needed. I figured we may as well do these updates together.

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CI testing TODO(s):

• core MSRV failure (expected) - requires core MSRV bump to 1.41 - already resolved by PR: Bump MSRV of core crates to 1.56.0 #2906
• clippy failure - already resolved by PR: resolve unnecessary map_or Clippy issue #2904
• general msrv failure - as resolved by PR: Bump general MSRV to 1.63 #2907

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other TODO(s):

• remove std option from futures-core & futures-sink - this would be another breaking change - I would be happy to do this here or in a separate PR