fix indenting
prepare v4 release
fix deficiencies in array_build

Copy values indirectly by index, giving us support for:

- associative arrays
- sparse arrays
- empty strings as elements
rename pkgbuild_introspection{,.inc.sh}
optimize array_build, add an error condition

Factor out the many evals into a single one by writing a small bash
program of printf -v calls which construct the new array. eval'ing
this at once brings us roughly back to the performance of the intial
single-line 'eval' statement.