Add prelude

[msrd0]

This PR adds a prelude module that pub uses some traits and all derive macros from gotham that define methods commonly used by user code.

It might make sense to also add functions, structs, re-exported crates etc to the prelude in the future.

[codecov]

Codecov Report

Merging #567 (9dde4de) into master (ca0f906) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
- Coverage   85.95%   85.88%   -0.07%     
==========================================
  Files         112      112              
  Lines        5781     5781              
==========================================
- Hits         4969     4965       -4     
- Misses        812      816       +4     

Impacted Files	Coverage Δ
examples/cookies/introduction/src/main.rs	88.88% <ø> (ø)
examples/custom_service/src/main.rs	2.77% <ø> (ø)
examples/diesel/src/main.rs	88.09% <ø> (ø)
...les/example_contribution_template/name/src/main.rs	76.19% <ø> (ø)
examples/finalizers/src/main.rs	85.71% <ø> (ø)
examples/handlers/async_handlers/src/main.rs	91.24% <ø> (ø)
examples/handlers/form_urlencoded/src/main.rs	83.33% <ø> (ø)
examples/handlers/multipart/src/main.rs	76.19% <ø> (ø)
examples/handlers/request_data/src/main.rs	88.88% <ø> (ø)
...xamples/handlers/simple_async_handlers/src/main.rs	90.00% <ø> (ø)
... and 31 more

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