Lines and Join are the same rule, so they are exported rather than copied

[tannevaled]

go-pdfkit/document has to know where
a paragraph ends and which line is a heading, and both questions start from the
lines of a page.

Writing a second line-grouping rule there would mean two rules that must
agree about what a page says, with nothing to make them. So Text's own rule
is exported instead:

lines := extract.Lines(runs)     // the runs gathered into their lines
s     := extract.Join(lines[0])  // one line's pieces, spaced as words

The test is the agreement itself — Lines joined with Join must read exactly
what Assemble reads. A copy could pass its own tests and still disagree.

Two one-line wrappers over groupIntoLines and joinLine, which already did
this for Text. 100% of statements, no uncovered blocks.

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