[config] nested `runs`

[tristan-f-r]

Currently, runs are specified per-algorithm as so:

run1:
  a: 5
  b: 6
run2:
  a: 4
  b: 5

Instead, we should organize this to be nested inside runs for better compatibility with keys we may add to algorithm (e.g. include) in the future.

runs:
  run1:
    a: 5
    b: 6
  run2:
    a: 4
    b: 5

This has parsing (no extraneous key support) and readability benefits.