WIP: remove StateData trait

[lucacasonato]

Closes #487.

This PR is still WIP, because documentation has not been updated yet. If this RFC is accepted, I will update the related docs.

[codecov]

Codecov Report

Merging #488 into master will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   84.90%   85.04%   +0.14%     
==========================================
  Files         109      108       -1     
  Lines        5599     5598       -1     
==========================================
+ Hits         4754     4761       +7     
+ Misses        845      837       -8     

Impacted Files	Coverage Δ
gotham/src/extractor/path.rs	66.66% <ø> (ø)
gotham/src/extractor/query_string.rs	66.66% <ø> (ø)
gotham/src/handler/assets/mod.rs	92.61% <ø> (ø)
gotham/src/middleware/session/mod.rs	79.34% <ø> (ø)
gotham/src/middleware/state/mod.rs	100.00% <ø> (ø)
gotham/src/pipeline/mod.rs	98.07% <ø> (ø)
gotham/src/router/builder/associated.rs	48.88% <ø> (ø)
gotham/src/router/builder/mod.rs	97.17% <ø> (ø)
gotham/src/router/builder/single.rs	94.11% <ø> (ø)
gotham/src/router/route/dispatch.rs	94.54% <ø> (ø)
... and 24 more

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