Beat-oriented functionality

[yaxu]

This PR allows timeCat("a b", "c d e") to mean the same as "a b c d e", and polymeter("a b", "c d e") the same as "{a b, c d e}" via the weight property of patterns.

There is also a new __pure property, so that 'pure' patterns can be treated as values by some functions.

Plus a new experimental beatCat function, which is similar to timeCat, but if an argument is a list, the sequence will repeat once for each element in the list.

[yaxu]

This is towards #987