[Breaking change] Made all decode_from_slice also return the number of bytes read

[VictorKoenders]

Currently there is no way to know how many bytes are read when decoding from a slice. This will make it harder to read multiple entries from a blob without encoding the length. This PR makes all decode_from_slice functions return the amount of bytes read.

This is a breaking change, but we're still on alpha so this should be okay.

Also fixes some of the new clippy warnings. This will conflict with #443 but we'll fix that when we get there

[codecov-commenter]

Codecov Report

Merging #445 (8fca53f) into trunk (240558d) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            trunk     #445      +/-   ##
==========================================
+ Coverage   62.87%   63.12%   +0.24%     
==========================================
  Files          46       46              
  Lines        3456     3479      +23     
==========================================
+ Hits         2173     2196      +23     
  Misses       1283     1283              

Impacted Files	Coverage Δ
derive/src/parse/generics.rs	62.45% <ø> (ø)
src/de/impl_core.rs	53.57% <ø> (ø)
src/de/read.rs	70.37% <ø> (ø)
src/features/serde/de_owned.rs	0.00% <ø> (ø)
src/lib.rs	100.00% <100.00%> (ø)
tests/alloc.rs	100.00% <100.00%> (ø)
tests/basic_types.rs	99.09% <100.00%> (+0.06%)	⬆️
tests/derive.rs	100.00% <100.00%> (ø)
tests/issues/issue_431.rs	100.00% <100.00%> (ø)
tests/std.rs	100.00% <100.00%> (ø)
... and 1 more

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[ZoeyR]

I don't really like returning the tuple, but the only alternative would be to make the decode_from_slice function take a mutable reference, which is more restrictive than it need be.