enable number validity checking and ptr::invalid checking by default by RalfJung...

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This removes the -Zmiri-check-number-validity flag, enabling its effects by default. (We don't error when the flag is passed, for backwards compatibility.) We also enable by default that transmuting an integer to a pointer now creates a pointer with None provenance, which is invalid to dereference (and, in the case of a function pointer, invalid to call). I did this together since it is all related to ptr2int/int2ptr transmutation.

Two new flags are added to optionally take back these stricter checks:

• -Zmiri-allow-uninit-numbers makes Miri accept uninit data in integers and floats
• -Zmiri-allow-ptr-int-transmute makes Miri accept pointers (provenance data) in integers and floats, and makes Miri treat int2ptr transmutes as equivalent to a cast.

The flag names make sense IMO, but they are somewhat inconsistent with our existing flags since we usually call things -Zmiri-disable-$CHECK rather than -Zmiri-allow-$THING. But -Zmiri-disable-uninit-number-check sounds silly?

(Whenever I say "transmute" this includes union and pointer based type punning.)
Cc @saethlin I hope this won't break everything?^^ I think the most risky part is the int2ptr transmute aspect, in particular around function pointers where no as casts are possible. The correct pattern is to first cast to a raw ptr and then transmute that to a fn ptr. We should probably document this better, in the transmute documentation and maybe in the documentation for the fn() type. I should run this PR against the std test suite before we land it.
r? @oli-obk

• Ensure stdlib docs recommend "usize -> raw ptr -> fn ptr" for int-to-fnptr casts: rust-lang/rust#97321
• Run the stdlib test suite