Handle varargs correctly in value_and_pullback!! and value_and_gradient!!

[willtebbutt]

Addresses #273 .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/interface.jl	91.66% <100.00%> (+0.23%)	⬆️

[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 │    107.0 │     1.0 │        5.61 │     1.9 │
│                  _sum_1000 │     7.74 │  1330.0 │        34.3 │   0.106 │
│               sum_sin_1000 │      2.5 │    1.61 │        10.9 │    1.02 │
│              _sum_sin_1000 │     3.11 │   303.0 │        17.6 │    1.48 │
│                   kron_sum │     62.2 │    10.3 │       263.0 │    10.6 │
│              kron_view_sum │     28.4 │    3.86 │        88.0 │    4.75 │
│      naive_map_sin_cos_exp │     3.18 │ missing │        9.05 │    2.83 │
│            map_sin_cos_exp │     4.72 │    1.76 │         8.2 │    3.41 │
│      broadcast_sin_cos_exp │      4.3 │    3.62 │        1.68 │    2.83 │
│                 simple_mlp │     8.72 │    3.39 │        14.5 │    3.37 │
│                     gp_lml │     14.9 │    6.55 │     missing │ missing │
│ turing_broadcast_benchmark │     7.25 │ missing │        34.8 │ missing │
│         large_single_block │      3.9 │  4260.0 │        31.1 │    2.24 │
└────────────────────────────┴──────────┴─────────┴─────────────┴─────────┘