implement pat or pattern instruction

[jmettraux]

pat a  # will return true if the workitem has an "a" field
pat b.c  # will return true if the workitem has a "b.c" field
pat b.c: true  # will return true if the workitem's b.c field holds `true`
pat !d  # will return true if the workitem doesn't have a d field
pat e, v.a  # will return true if the workitem has a e field and the variable a is set
# ...

[jmettraux]

... well, it's just an and ...