Add drain method to Storage

[torkleyy]

Fixes #264

[torkleyy]

r? @malleusinferni @Aceeri

[torkleyy]

I'm not sure why coverage goes down.. does it really want me to test drain for every type of storage?

[WaDelma]

You should be able to make coveralls to show the file and which lines are covered by test.
Login on coveralls using github, click file and it should let you fill the where the sources are (which is src for us=

[torkleyy]

The problem is that the coverage is inaccurate. E.g. I had to test derived traits, but that doesn't make any sense. I ran the coverage locally and it was not obvious what test I should add, see Gitter.

[malleusinferni]

Looks good to me. I tried it out with a few reduced test cases and it does exactly what I was asking for. Thanks @torkleyy!

[kvark]

this is pretty cool!
ready to merge?

[torkleyy]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push