Groups async implicits into AsyncInstances trait

[juanpedromoreno]

This PR groups the set of async common instances (Future, Task) into the AsyncInstances trait. Then these implicits are provided for both server and client implicit imports:

import freestyle.rpc.server.implicits._ or import freestyle.rpc.client.implicits._.

Fixes #70
Fixes #66

[codecov-io]

Codecov Report

Merging #71 into master will decrease coverage by 0.85%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   89.38%   88.52%   -0.86%     
==========================================
  Files          12       12              
  Lines         113      122       +9     
==========================================
+ Hits          101      108       +7     
- Misses         12       14       +2

Impacted Files	Coverage Δ
rpc/src/main/scala/server/implicits.scala	100% <ø> (ø)	⬆️
rpc/src/main/scala/AsyncInstances.scala	80% <80%> (ø)

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[eperinan]

LGTM, and really useful this change !