Fixed an error in bincode derive where it would implement the wrong trait if a generic parameter is present

[VictorKoenders]

For any enum with generic argument T, bincode-derive would implement a code like:

impl<T> bincode::Decode for ... where T: bincode::enc::Decode {

[codecov]

Codecov Report

Merging #487 (30423dd) into trunk (919ac4f) will increase coverage by 0.53%.
The diff coverage is 98.30%.

@@            Coverage Diff             @@
##            trunk     #487      +/-   ##
==========================================
+ Coverage   68.73%   69.26%   +0.53%     
==========================================
  Files          39       39              
  Lines        2875     2925      +50     
==========================================
+ Hits         1976     2026      +50     
  Misses        899      899              

Impacted Files	Coverage Δ
derive/src/derive_enum.rs	0.00% <0.00%> (ø)
tests/alloc.rs	98.38% <ø> (ø)
tests/std.rs	100.00% <ø> (ø)
tests/utils.rs	31.81% <ø> (ø)
tests/derive.rs	100.00% <100.00%> (ø)

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