Cost tracking for two-variable comprehensions, cel.bind(), and optional_types

This update introduces the concept of a computed entry size for literals which helps
with cost propagation through chained computations. The cost-tracking has cannot
estimate further than one level of nesting within a literal value, but this should be suitable
for most cases.

This change also perform optional and dyn unwrapping, de-emphasizes dynamic size
computations by aggressively caching values though this be improved further for local
variables which require calls into the estimator.

Note, the cost tracking beyond a single level of depth for a literal is quite limited, but considerably improved from its former state.