Source-to-source

[willtebbutt]

Work on source-to-source AD.

ToDo:

1. review PR and ensure that everything is tidy + documented + redundant code has been removed
2. update readme to reflect the state of the package, and to make it clear how to get started

[github-actions]

Performance Ratio:

┌────────────────────────────┬────────┬─────────┬─────────────┬─────────┐
│                      Label │  Taped │  Zygote │ ReverseDiff │  Enzyme │
│                     String │ String │  String │      String │  String │
├────────────────────────────┼────────┼─────────┼─────────────┼─────────┤
│                        sum │   98.1 │   0.332 │        2.24 │   0.517 │
│                       _sum │   25.9 │   502.0 │        32.9 │   0.121 │
│                   kron_sum │  196.0 │    3.65 │       219.0 │    24.2 │
│              kron_view_sum │  241.0 │    10.8 │       242.0 │    8.26 │
│      naive_map_sin_cos_exp │   8.48 │ missing │        8.73 │    2.75 │
│            map_sin_cos_exp │   11.0 │    1.75 │        7.33 │    3.39 │
│      broadcast_sin_cos_exp │   9.83 │    2.63 │        1.66 │    2.83 │
│                 simple_mlp │   23.4 │    3.07 │        11.8 │    3.68 │
│                     gp_lml │   43.4 │    4.29 │     missing │ missing │
│ turing_broadcast_benchmark │   19.9 │ missing │        34.0 │ missing │
└────────────────────────────┴────────┴─────────┴─────────────┴─────────┘