as_label() and expr_text() should use expr_deparse()

[hadley]

Instead of deparse

[lionel-]

It is probably too early to switch to expr_deparse() right away. I've just fixed two bugs while trying to switch: fa47703 and 6a2b6bb. Since expr_label() is now used in testthat we should test-drive the deparsing algorithm for a while.

Also there's the performance issue. expr_deparse() is very slow because:

• it's a recursive function written in R;
• it reallocates a new string each time each new characters are added to the last line and a whole new character vector of lines each time a new line is added (which is likely costly even if it's only a vector of pointers to interned strings).

Once it's feature-complete it should be rewritten in C with an extensible buffer, perhaps with a non-recursive algorithm.