[hail] Improve performance of IR copying

[tpoterba]

• don't use inefficient unapply
• Cache the returnType of AggSignatures so they persist through copies

                           Name      Ratio    Time 1    Time 2
                           ----      -----    ------    ------
table_big_aggregate_compilation      85.6%     4.113     3.519
----------------------
Geometric mean: 85.6%
Simple mean: 85.6%
Median:  85.6%

[tpoterba]

Prune and ForwardLets probably need to be adjusted to allow no-copy passes if nothing changes.

[iitalics]

doesnt NDArrayShape need an assert?

[tpoterba]

added asserts.