async-sse doesn't use any of the features of http-types, so don't pull
them in unless something else needs them.

It may be useful to be able to use this in more ecosystems without
taking the whole async-std dependency. The async-std dependency was
mostly through re-exports that can instead be taken directly on the
futures crate.

Currently, the `data` passed to `Sender::send()` is prepended with
`data:` and written as-is. If the data string contains newlines, that
causes any data beyond the newline(s) to end up on a line that does not
begin with `data:`, which the spec dictates should be ignored. This
change splits the message on newlines and writes each line with its own
`data:` prefix.

deps: replace async-std with futures-lite

Updated pin-project-lite to 0.2.7

add support for multiline data messages

Depend on http-types without default-features