Some small improvements to low-level rules

[willtebbutt]

There are some low-level functions which ought not to have rules, because there's perfectly good low-level functionality which handles them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/rrules/low_level_maths.jl	100.00% <100.00%> (ø)

[github-actions]

Performance Ratio:
Ratio of time to compute gradient and time to compute function.
Warning: results are very approximate! See here for more context.

┌────────────────────────────┬──────────┬─────────┬─────────────┬─────────┐
│                      Label │ Mooncake │  Zygote │ ReverseDiff │  Enzyme │
│                     String │   String │  String │      String │  String │
├────────────────────────────┼──────────┼─────────┼─────────────┼─────────┤
│                   sum_1000 │    113.0 │     1.0 │        5.41 │     1.9 │
│                  _sum_1000 │     7.79 │  1290.0 │        33.6 │   0.106 │
│               sum_sin_1000 │     2.61 │    1.59 │        11.1 │   0.995 │
│              _sum_sin_1000 │     2.93 │   304.0 │        16.5 │    1.42 │
│                   kron_sum │     62.6 │    10.5 │       199.0 │    11.8 │
│              kron_view_sum │     29.5 │    4.01 │        76.4 │    2.97 │
│      naive_map_sin_cos_exp │     3.14 │ missing │        9.68 │    2.77 │
│            map_sin_cos_exp │     4.83 │    1.77 │         7.8 │    3.39 │
│      broadcast_sin_cos_exp │     4.34 │    4.63 │        1.67 │    2.83 │
│                 simple_mlp │     8.03 │    3.08 │        12.2 │    3.12 │
│                     gp_lml │     14.7 │    4.43 │     missing │ missing │
│ turing_broadcast_benchmark │     7.65 │ missing │        30.3 │ missing │
│         large_single_block │     3.89 │  4140.0 │        30.3 │    2.23 │
└────────────────────────────┴──────────┴─────────┴─────────────┴─────────┘