re-export rt in web and add main macro

[robjtede]

I made a mistake in #1559. The new example file compiled but only because actix-rt is still an explicit dependency of actix-web. This PR re-exports the bundled actix-rt as actix_web::rt, thus allowing access to actix_web::rt::System and copies the main macro out of actix-macros, modifying it to use the new re-export.

Also migrated the main lib.rs module doc comment to use the actix_web::main macro as well as some reduced the scope of some clippy lints.

[codecov-commenter]

Codecov Report

Merging #1575 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1575   +/-   ##
=======================================
  Coverage   52.44%   52.44%           
=======================================
  Files         122      122           
  Lines       11630    11630           
=======================================
  Hits         6099     6099           
  Misses       5531     5531           

Impacted Files	Coverage Δ
src/app.rs	98.99% <ø> (ø)
src/info.rs	92.85% <ø> (ø)
src/lib.rs	68.42% <ø> (ø)
src/middleware/compress.rs	76.27% <ø> (ø)
src/middleware/defaultheaders.rs	85.48% <ø> (ø)
src/route.rs	85.88% <ø> (ø)
src/types/form.rs	77.24% <ø> (ø)
src/types/json.rs	88.71% <ø> (ø)
src/types/payload.rs	81.34% <ø> (ø)
src/middleware/logger.rs	74.79% <100.00%> (ø)

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[cetra3]

Yeah this change looks fine if it's all working now