Fix isassigned for BitArrays

(and therefore show_delim_array, show_vector, showall, repr,
the show macro etc.)

Fixes #5754
SharedArray: don't splat for getindex/setindex! dimensions 1-5

Yields a ~10x performance increase for 3-dimensional SharedArrays
Better fix for #5754

Split show_delim_array in two, one version specialized for
AbstractArrays which uses linear indexing and checks for undefs,
and a generic one which uses start/next/done.

Also properly implement isassigned for BitArrays.
setfield -> setfield