rust: replace most "operation" structs with functions The hg-core crate has a partially-formed concept of "operation", represented as structs with constructors and a `run` method. Each struct’s contructor takes different parameters, and each `run` has a different return type. Constructors typically don’t do much more than store parameters for `run` to access them. There was a comment about adding an `Operation` trait when the language supports expressing something so general, but it’s hard to imagine how operations with such different APIs could be used in a generic context. This commit starts removing the concept of "operation", since those are pretty much just functions. Differential Revision: https://phab.mercurial-scm.org/D9595