Make SparseBitMatrix a bit lazier.
Make SparseBitMatrix a bit lazier.

Currently when a row is instantiated in SparseBitMatrix, any missing
rows prior to it are also fully instantiated.

This patch changes things so that those prior rows are minimally
instantiated (with a `None`). This avoids a decent number of allocations
in NLL, speeding up several benchmarks by up to 0.5%.

The patch also removes two unused methods, `len()` and
`iter_enumerated()`.

GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust