Pre-compiled Declarative Modules

[lihaoyi]

This PR allows users to define module traits that can be instantiated in build.mill.yaml and package.mill.yaml files without needing to be codegened and compiled. This can speed up Mill initialization substantially in scenarios where the build is mostly declarative .mill.yaml modules.

The user opts in by defining a class PreCompiled() extends mill.api.PreCompiledModule in their trait's companion object, and any .mill.yaml file that extends a single label that matches that class will use the pre-compiled class instantiated reflectively rather than the original trait instantiated via codegen.

Notes:

• We are only able to skip codegen in yaml modules who do not have programmable descendents. For programmable modules with yaml descendents, we would need to codegen the yaml module reference in the nearest enclosing programmable ancestor, so build.foo.bar.qux programmable references works.

• So if everything is declarative then no codegen needs to happen, but if the root is programmable and the children are declarative the root needs to generate a object tree, and if the root is declarative and some descendent is programmable we also need to codegen something for the root to handle build.* references