The Vec type is a generalization of Buffer. In particular, this PR
exposes an API to create and manage Vec<Tag> and replaces existing uses
of Buffer with Vec<u8>. It includes a helper to parse DD_TAGS as well.

One of the APIs, ddprof_ffi_Buffer_reset, was removed. Previously this
was required when using the failure case of the SendResult type, but
now there is ddprof_ffi_SendResult_drop, which should be used instead.

Previously it couldn't handle a tags like:
  env:staging:east
  value

Tag messages on failure have improved.

As part of adding tests, added more trait support to Slice and
CharSlice so they can be used in equivalence statements.

As part of doing that, I realized Slice could implement IntoIterator
and that it cleaned up 3+ places where we call .into_slice().iter().
I also removed unsafe from `into_slice`. Technically, it could still
fail such as if the user creates a slice of length 4 but only has 2
elements, but this is a user error that cannot really be checked.