Store the backtrace as a series of records rather than strings

We take the hit of parsing the OCaml < 4.02 stacktraces safe in the
knowledge that we can optimise this away (with optcomp) later.

This also means we take full control over the rendering of stacktraces
and can make the python and OCaml backtraces look the same.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add CHANGES

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #4 from djs55/improve-api

Store backtraces as lists of records rather than strings