testsuite: Add mechanism to collect generic metrics

* Generalise the metric logic by adding an additional field which
  allows you to specify how to query for the actual value. Previously
  the method of querying the baseline value was abstracted (but always
  set to the same thing).

* This requires rejigging how the stat collection works slightly but now
  it's more uniform and hopefully simpler.

* Introduce some new "generic" helper functions for writing generic
  stats tests.

  - collect_size ( deviation, path )
    Record the size of the file as a metric

  - stat_from_file ( metric, deviation, path )
    Read a value from the given path, and store that as a metric

  - collect_generic_stat ( metric, deviation, get_stat)
    Provide your own `get_stat` function, `lambda way: <Int>`, which
    can be used to establish the current value of the metric.

  - collect_generic_stats ( metric_info ):
    Like collect_generic_stat but provide the whole dictionary of metric
    definitions.

    { metric: {
        deviation: <Int>
        current: lambda way: <Int>
        } }

* Introduce two new "size" metrics for keeping track of build products.
    - `size_hello_obj` - The size of `hello.o` from compiling hello.hs
    - `libdir` - The total size of the `libdir` folder.

* Track the number of modules in the AST tests
   - CountDepsAst
   - CountDepsParser

This lays the infrastructure for #24191 #22256 #17129