Fine-grained rules: third iteration of the design

This commit introduces an update of the design of fine-grained rules.

Main changes to the design:

  - rules that depend on the output of another rule must depend on its
    `RuleId`, and not simply on the path of the output of the rule,
  - all searching happens in rule generation instead of in the build
    system,
  - users are required to define rule names which are used to register
    rules, with the API returning an opaque `RuleId` (with additional
    namespacing to ensure they can be combined across packages,
    using static pointers),
  - use of static pointers for actions, which allows us to get rid of
    `ActionId` and to get rid of the doubly-nested monadic structure;
    this improves debuggability (as we will show rule names that are
    meaningful to the user),
  - let the user be in control of what data is passed to the action
    that executes a rule,
  - removal of the rule `monitoredValue` (made redundant by `Eq Rule`
    instance)
  - additional functionality for declaring extra dynamic dependencies
    of rules, to avoid recomputing rules entirely when e.g. a `.chs`
    file is modified.

This commit also fleshes out the justification of the design,
comparing with Shake and ninja, and adds a bit more context about the
requirements imposed by the IPC interface.

Author: sheaf