I made the "tuple structs are useless" editorializing a bit weaker and moved it to the end. Feel free to overrule me on that.
I also added an example of how to unpack a tuple struct with dot notation, because it came up on IRC.
`braced_empty_structs` is stable now, so I updated the example for unit-like structs to use that syntax. Should we show both ways?
cc @ubsan
r? @steveklabnik or @GuillameGomez
The book was located under 'src/doc/trpl' because originally, it was
going to be hosted under that URL. Late in the game, before 1.0, we
decided that /book was a better one, so we changed the output, but
not the input. This causes confusion for no good reason. So we'll change
the source directory to look like the output directory, like for every
other thing in src/doc.
