Create a common method for skipping variable references

Create a common method for skipping variable references

* README.git: Add some notes about using ASAN.
* src/makeint.h: Declare skip_references().
* src/misc.c (skip_reference): A new function that will skip over a
variable reference, counting matching open paren/brace characters.
* src/implicit.c (get_next_word): Replace code with skip_reference().
* src/read.c (conditional_line): Ditto.
(find_map_unquote): Ditto.
(get_next_mword): Ditto.
(parse_variable_definition): Ditto.
* src/function.c (handle_function): Make clear that the passed in
pointers are not modified if the function returns false.
* src/expand.c (expand_string_buf): Don't create local variables to
call handle_function() since it doesn't modify its arguments.
* src/job.c (new_job): Small simplifications.