Optimize counting digits in line numbers during error reporting
Replaces `.to_string().len()` with simple loop and integer division, which avoids an unnecessary allocation.
Although I couldn't figure out how to directly profile `rustc`'s error reporting, I ran a microbenchmark on my machine (2.9 GHz Dual-Core Intel Core i5) on the two strategies for `0..100_000`, and the results seem promising:
```
test to_string_len ... bench: 12,124,792 ns/iter (+/- 700,652)
test while_loop ... bench: 30,333 ns/iter (+/- 562)
```
The x86_64 disassembly reduces integer division to a multiplication + shift, so I don't think there's any problems with using integer division.
For more (micro)optimization, it would be nice if we could avoid the initial check to see if the line number is nonzero, but I don't think `self.get_max_line_num(span, children)` _guarantees_ a nonzero line number.
555db2da70