Introduce IR for analysis

[axic]

Or at least helpers with some data structures.

A good start would be doing a CFG (control flow graph) or at least a primitive version which shows dependencies of functions.

This can be used in two ways in the short term:

• finding unused functions
• in "atomic metering" to find the leaf nodes to start metering on

[axic]

Could consider using https://github.com/rustwasm/walrus because it has the same goals.