[Experimental] Split out an `UnsafeCoerceUnsized` trait from `CoerceUnsized`#88239

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crlf0710:unsafe_coerce_unsized

Commits on Aug 22, 2021

Add the `UnsafeCoerceUnsized` lang-item trait and migrate all existing usages.
crlf0710
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Implement unsafety checking for unsizing coercion.
crlf0710
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