Easy mechanism to disable noisy fails.

This cute diff mechanism auto runs even if for expected-fail buffers
which is irritating.
Edebug Spec for Macro.
Now uses random face.

Previously, we used a named list and rotated through them, but in actual
use we don't care about a predictable order, just about having a
different face every time, so this works better.
Moved with-markers functionality to m-buffer.
Specialised beginning of line treatment.

The beginning of line is now treated specially to cover changes being
made to the comment section in linked-buffer-block. Previously
disruption to the comments would not percolate correctly.

The converted start and stop values are now passed explicitly, rather
than hidden in the conf object which makes this easier.