Add actx.outline decorator

cosmetic-ish refactor of OutlinedCall.__call__

includes minor change to handle array containers that contain scalars
(e.g., a TracePair of
constant diffusion coefs)

check for non-argument placeholders in outlined function

drop unused function arguments

pass hashable instead of string as id to outline

handle optional arguments that are passed as None explicitly

change Map -> immutabledict in outlining

remove duplicates when creating FunctionDefinition

add comment explaining non-argument placeholder check

Co-authored-by: Matthew Smith <mjsmith6@illinois.edu>
Co-authored-by: Andreas Kloeckner <inform@tiker.net>