Support two-element slicing with literal None

Closes #2587.

Simplify encoding detection and source file opening by starting off reading bytes, and only switching to decoding once we know which encoding to use.

Further experimentation - leaks references, C compilation raises warnings, C++ compilation fails outright.

Reworking slicing unittests to include sliced type - added unsuccessful attempt to resolve issue #2508.

Print run time stats in the test runner

Split C/C++ tests in travis again, but manually this time since many of the build matrix configs are special anyway.