LineProgram::lines_at_offset is pretty slow

[calixteman]

Looking at https://github.com/willglynn/pdb/blob/master/src/modi/c13.rs#L897 it seems that getting all the line info for all the files is quadratic in the number of files.

[jan-auer]

Correct. It currently parses all sections over and over again, instead of just once when constructing the line program.

I have a preliminaty fix for this already, will try to check it in soon.

[calixteman]

It'd be nice :)
Using perf I noticed that 20% of the time is spent in this fonction.