Replace [] syntax with .as_slice()

Latest nightly seems to have removed the [] syntax for complete slices ([a:b] doesn't work either). The new syntax is a bit more verbose but also much more clear.

This should be merged once current nightly becomes the stable release.
Tested with `rustc 0.12.0-nightly (af3889f69 2014-09-18 21:20:38 +0000)`
Closes #6.