Make `from_bits` in `bitflags!` safe; add `from_bits_truncate`

Previously, the `from_bits` function in the `std::bitflags::bitflags`
macro was marked as unsafe, as it did not check that the bits being
converted actually corresponded to flags.

This patch changes the function to check against the full set of
possible flags and return an `Option` which is `None` if a non-flag bit
is set. It also adds a `from_bits_truncate` function which simply zeros
any bits not corresponding to a flag.

This addresses the concern raised in https://github.com/mozilla/rust/pull/13897

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