Try to ensure that using "the width computed during an unconstrained layout" as the width constraint during a relayout produces the same layout. Note, here I aim to also make this work for cases where the initial width limit isn't completely unconstrained. This is useful for certain UI layout algorithms.

Fixes #134

• Instead of computing the LayoutLine width from the positioned and aligned glyphs, we pass through width computed during line wrapping (unless justified alignment is used, in this case we use the old approach because the use case for measuring the width isn't really applicable to justified text since that will just expand to the provided width). For the produced width to later give the same wrapping results when passed in as the line_width it needs to use the same exact float arithmetic that was used to compute the width that is compared against line_width when making line wrapping choices. Passing this width through as the LayoutLine width is the most convenient option without making more major changes to the API. Nevertheless, I am imagining that if we get a dedicated measurement method (i.e. that doesn't do the final positioning and alignment of glyphs and which caches Vec<VisualLine>), then this width can just be exposed there instead of preserving it in LayoutLine.
  edit: one thing that I was going to mention here is that this means the width in LayoutLine might not as precisely match the positioned glyphs where their positions have been perturbed by different floating point errors.
• Incidentally, this fixes Line width measurement different when aligned #169. Note: If that behavior was intentional, then a different approach is needed to expose the width to the user.
• Switch subtraction from fit_x to checking whether potential addition to the current line width would exceed the line_width. This avoids the float error being dependent on the provided line_width value.
• When eliminating trailing space from the line width, we avoid backtracking with subtraction (which would not give the same exact value due to float error) and instead save the previous width and use that.
• If the previous word did not exceed the line_width, we now include a single blank word even if it would cross the width limit since its width won't be counted. This is necessary to get the same wrapping behavior when re-using the measured width (which doesn't count a single trailing blank word). Note, this whitespace logic may be reworked anyway if Wrap::Word inserts newline for trailing space (after word) #155 is addressed.
• Change tests to use opt-level=1 to keep test runtime down.
• Add fonts folder for fonts used in tests.
• Fix an issue where a non-breaking whitespace was assumed to be the start of a section of spaces which included characters that weren't even whitespace.
• Add some TODOs about incongruencies between is_whitespace, justification, and line breaks.