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

Use debug builder on stable and bump version

Use the stabilized API (from 1.5.0) to compare iterators.

Use the stabilized API (from 1.5.0) to compare iterators. Keep nightly attributes since the Rust compiler is still at 1.4.0.

replace uses of removed `std::hash::hash` in tests

Assorted updates