Refactor the @service macro

[cb372]

Split the monolithic macro implementation into a few files.

Note: to help the compiler out with the path-dependent types arising from use of Context, the helper classes must be of the form

class MyHelper[C <: Context](val c: C)

and when you instantiate the helper, you need to give it the Context instance's singleton type:

val myHelper = new MyHelper[c.type](c)

Ideally I'd like the Operation, RPCMethod and HttpOperation classes to be in separate files, but I started to hit the limits of scalac's path-dependent types support so I had to group them together.

Potentially a few more utility methods could be moved to the TreeHelpers class, but this refactoring is a good start.

Also tidied up the HTTP-related macro code:

• removed an unnecessary line from the val httpOperations for-comprehension
• fixed some code that relied on the service definition trait's type parameter being called F[_] (the macro would have failed if the user decided to name their type param G[_])

Fixes #874, fixes #844

[codecov]

Codecov Report

Merging #887 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #887   +/-   ##
=======================================
  Coverage   86.28%   86.28%           
=======================================
  Files          57       57           
  Lines         824      824           
  Branches        5        5           
=======================================
  Hits          711      711           
  Misses        113      113           

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[BenFradet]

Feels less clunky 👍

[juanpedromoreno]

👌

[juanpedromoreno]

Seems like the four options here are identical, except the clientMethodName, maybe we can simplify the pattern matching to get that information and leave the rest outside just once?