Eventually we would like all our crates other than `bitcoin` to be able
to be used without an allocator. Currently, and during crate smashing,
this is not that useful because so much of the code comes from `bitcoin`
and relies on the availability of an allocator.
As an initial step, add the `alloc` feature to `addresses` , `base58`,
and `primitives`.
In order to to keep `--no-default-features` builds working make the
crates empty if the `alloc` feature is not enabled. This is a suboptimal
solution because the error messages users will get when they forget to
enable `alloc` will be confusing (eg something like primitives does not
contain Transaction). However our CI script (`run_task.sh`) expects
`--no-default-features` to build cleanly (as do I).
We intend on splitting the address types and logic out into a separate
crate. In preparation for doing so, and so that we can grab the name on
crates.io, add an empty crate `bitcoin-addresses`.
Tie it in to the CI infrastructure.
Tobin C. Harding