reflect: add TypeAssert[T]

[mateusz834]

This implementation is zero-alloc, except when a Value is
a method, in that case it allocates.

BenchmarkTypeAssertTime-12 257062033 4.692 ns/op

Fixes #62121

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Tryjob golang/try/gotip-linux-amd64-misccompile has failed with summary (view all results):

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Error:

shard 4: failed to run compile goos:"freebsd" goarch:"386" port: exit status 1
failed to run compile goos:"freebsd" goarch:"amd64" port: exit status 1
failed to run compile goos:"freebsd" goarch:"arm" port: exit status 1
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• Output for compile goos:"freebsd" goarch:"arm" port [shard 4]
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