add clippy linting CI target

[danieleades]

adds a clippy linting CI target.

Couple of questions on this-

• what is the best possible combination of features to use for linting?
• would you prefer to have the compiler version used for clippy linting be pinned? (that prevents build failures on PRs due to new lints, but means the version must be manually updated at intervals)

[codecov]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (cc223f5) 51.49% compared to head (ac9e802) 51.55%.

Files	Patch %	Lines
capnp-rpc/src/rpc.rs	50.00%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   51.49%   51.55%   +0.05%     
==========================================
  Files          69       69              
  Lines       33732    33733       +1     
==========================================
+ Hits        17371    17390      +19     
+ Misses      16361    16343      -18     

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[danieleades]

@dwrensha this is complete and ready for review.

Note that to get the linting to pass i've had to temporarily suppress a type complexity lint.
I'm not in a position to create the type aliases to make this lint happy, as the types are impenetrable to me (which illustrates the value of the lint!)
I leave that as an exercise ;)

[dwrensha]

Why commented out?

[danieleades]

as above. this should be uncommented as soon as the type complexity lint is addressed.

Would you prefer i remove the commented sections?