Memory and Time Cost Estimators

[sg495]

Desired features:

• Implement memory size estimates for boxclasses, based on shape only.
• Implement time cost and intermediate memory size estimates for each of the basic contraction ops that can be defined by a boxclass: contract2, rewire and spider.
• Implement time cost and intermediate memory size estimates for boxclass casts (cf. Boxclass Casts #8).

Additionally, it will be necessary to either (i) mandate that contraction operations be final, or (ii) make the estimates compatible with inheritance and method overrides.