Make debug info available programatically

[nrc]

Probably using macros.

Tracking issue for #468

cc @mitsuhiko (I hope I have got the title about right as a slightly more general version of your RFC, let me know if you think there should be a better title.

[mitsuhiko]

There are two parts:

• the ability to cheaply freeze a debug location (for instance the program counter) for later inspection.
• the ability to resolve the frozen information into debug information.

Not sure what the correct title for this is.