exclude unneeded files from crates.io

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

Introduce feature "serde" for enabling serde serialization

Fix some compiler (Rust 1.26) and Clippy nags

The old feature "eders" is kept for backwards compability.

Fixes #38

There should be no functional change.

Add values_mut (fixes #36) and shrink_to_fit (fixes #28).

add retain method

Keep track of number of set elements in map

Bumped serde version from 0.9 to 1.0

This allows much more efficient versions of len() and is_empty(), and
also enables ExactSizeIterator to be implemented for VecMap's iterators.

Fixes #32

impl clone_from() for VecMap

bump serde version

Signed-off-by: Igor Gnatenko <ignatenko@redhat.com>

bump serde version

Assorted updates

Closes #13.

Adding nightly serde support for VecMap under "eders" feature flag

Adding eders feature for separate TravisCI nightly build.
Adding compilation test for serde traits.

Change interface to take usize