Small performance fix.

This makes bitvector push_back's amortized constant time. Earlier, push_back
boiled down to vector<T>::resize, which has complexity linear in the size of
the container.
Switch to boolean signal for flow control.
A few improvements to filesystem error reporting.
Logging and configuration nits.
Fix bug in string strimming.
Fix nasty bug in value parsing.

The temporary string object did not exist anymore when we iterated over the
split expression.