add function decorator with serialization cache

[joostvanzwieten]

When calling a decorated function all array arguments are replaced by placeholders. The function is serialized once for each unique set of placeholder arguments. Decorated functions behave exactly as undecorated functions provided that the function does not rely on values outside the function namespace, i.e. returns the same value whenever called with the same set of arguments.

[joostvanzwieten]

Superseded by nutils.function.Argument, introduced in #210.