Switched Decode and BorrowDecode to take &mut D

[VictorKoenders]

fixes #467

[codecov-commenter]

Codecov Report

Merging #470 (c4cb2d7) into trunk (6d38649) will decrease coverage by 0.09%.
The diff coverage is 89.92%.

@@            Coverage Diff             @@
##            trunk     #470      +/-   ##
==========================================
- Coverage   67.01%   66.91%   -0.10%     
==========================================
  Files          35       36       +1     
  Lines        2671     2672       +1     
==========================================
- Hits         1790     1788       -2     
- Misses        881      884       +3     

Impacted Files	Coverage Δ
derive/src/derive_enum.rs	0.00% <0.00%> (ø)
derive/src/derive_struct.rs	0.00% <0.00%> (ø)
src/features/serde/mod.rs	0.00% <0.00%> (ø)
src/de/impls.rs	79.22% <96.49%> (ø)
src/de/impl_tuples.rs	100.00% <100.00%> (ø)
src/de/mod.rs	65.51% <100.00%> (-13.80%)	⬇️
src/features/atomic.rs	100.00% <100.00%> (ø)
src/features/impl_alloc.rs	98.24% <100.00%> (ø)
src/features/impl_std.rs	77.55% <100.00%> (ø)
tests/alloc.rs	100.00% <100.00%> (ø)
... and 3 more

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