core: Remove the cast module

[alexcrichton]

This commit revisits the cast module in libcore and libstd, and scrutinizes
all functions inside of it. The result was to remove the cast module entirely,
folding all functionality into the mem module. Specifically, this is the fate
of each function in the cast module.

• transmute - This function was moved to mem, but it is now marked as
  #[unstable]. This is due to planned changes to the transmute
  function and how it can be invoked (see the #[unstable] comment).
  For more information, see RFC 5 and Check transmute restrictions in the crate where it is used #12898
• transmute_copy - This function was moved to mem, with clarification that is
  is not an error to invoke it with T/U that are different
  sizes, but rather that it is strongly discouraged. This
  function is now #[stable]
• forget - This function was moved to mem and marked #[stable]
• bump_box_refcount - This function was removed due to the deprecation of
  managed boxes as well as its questionable utility.
• transmute_mut - This function was previously deprecated, and removed as part
  of this commit.
• transmute_mut_unsafe - This function doesn't serve much of a purpose when it
  can be achieved with an as in safe code, so it was
  removed.
• transmute_lifetime - This function was removed because it is likely a strong
  indication that code is incorrect in the first place.
• transmute_mut_lifetime - This function was removed for the same reasons as
  transmute_lifetime
• copy_lifetime - This function was moved to mem, but it is marked
  #[unstable] now due to the likelihood of being removed in
  the future if it is found to not be very useful.
• copy_mut_lifetime - This function was also moved to mem, but had the same
  treatment as copy_lifetime.
• copy_lifetime_vec - This function was removed because it is not used today,
  and its existence is not necessary with DST
  (copy_lifetime will suffice).

In summary, the cast module was stripped down to these functions, and then the
functions were moved to the mem module.

transmute - #[unstable]
transmute_copy - #[stable]
forget - #[stable]
copy_lifetime - #[unstable]
copy_mut_lifetime - #[unstable]

[lilyball]

I think you accidentally a word.

[alexcrichton]

Updated with comments so far.