Fix various small issues in driver module.

[jneem]

This makes some fields public, so that it's possible to get interesting
information out of IoEvents. Also, fixes the behaviour of
ProcessingSessionBuilder::output_dir to match its documentation (i.e.,
don't panic if the output_dir is not provided).

[pkgw]

Looks great, will merge as soon as CI passes. Thanks!

[codecov]

Codecov Report

Merging #184 into master will increase coverage by 0.18%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   37.97%   38.15%   +0.18%     
==========================================
  Files         131      131              
  Lines       60830    60974     +144     
==========================================
+ Hits        23099    23264     +165     
+ Misses      37731    37710      -21

Impacted Files	Coverage Δ
src/driver.rs	52.83% <78.94%> (+15.35%)	⬆️

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