refactor "destinations"

now takes operation as arguments, if missing returns value:

destination               # $PWD
destination push $HOME    #
destination               # $HOME
destination pop
fn: destination returns 1 on unknown arg

this is just good pratice
fn: "destination pop" returns 1 on empty stack

hey, bash does this for us!
fn: arguments, laying groundwork
fn: destination gets size, clear ops

hopefully these will be useful beyond the test use cases
fn: destination, refactor
src: start of git source