add column location information to log statements

[japaric]

currently we only display file path and line for each log statement.

AFAICT, rustc doesn't emit DW_AT_decl_column DWARF info with debuginfo=2. So until it does this blocked on upstream support for that.

[japaric]

looked into this a bit more. in particular, I was wondering why backtraces have column information when DW_AT_decl_column info is not emitted. what I found out is that addr2line, which std uses under the hood, is using a different part of DWARF info to resolve a Program Counter (PC) address back into (inlined) functions, or rather statements within functions; DWARF contains location information, including column info, for statements.

defmt works with static variables addresses (those are the indices of defmt strings) rather than PC addreses. probe-run uses the DWARF to find the (file, line) location of those static variables. although it seems possible to use a statement-based approach like addr2line does to get column information for log statements that would be quite different from what we are doing now so likely lots of work for little gain. I would personally just wait for DW_AT_decl_column being added to the debuginfo fo Rust programs.