This removes about 200 pointless copies from printing the help
message (the original motivation for the change), and also nicely
improves performance of the various reporters that depend on
TextFlow.
The problem was that every line would iterate from current line
start position to the end of the string, looking for a newline
to break on, leading to accidentally quadratic runtime. With this
change, the code only ever searches up to the current line's
length and not more.
Credit to @jorgenpt for the fix suggestion.
Closes#2315
Now that it has its own header, various reporter TUs that want to
format text do not have to also include Clara. Together with
outlining implementations from a header into a separate TU, this
has noticeably improved the compilation times of the testing impl.
As part of this split, I also implemented some improvements to the
TextFlow code in comparison to the upstream code. These are:
* Replaced the `Spacer` type with a free function that constructs
special `Column` that does the same thing.
* Generic performance improvements, such as eliminating needless
allocations, reserving space in needed allocations, and using smarter
algorithms in some places.
* Because `Column` only ever stored 1 string in its vector, it now
holds the string directly instead.
Martin Hořeňovský
Lars Toenning