Update to Rust edition2018, fix compile warnings

[Patrik-Stas]

No description provided.

[codecov-io]

Codecov Report

Merging #204 (4450d17) into master (f06ba4c) will decrease coverage by 0.00%.
The diff coverage is 73.27%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   60.79%   60.79%   -0.01%     
==========================================
  Files         147      147              
  Lines       19648    19650       +2     
  Branches     4322     4322              
==========================================
+ Hits        11945    11946       +1     
- Misses       4255     4256       +1     
  Partials     3448     3448              

Flag	Coverage Δ
integration	35.45% <63.04%> (+<0.01%)	⬆️
unittests	53.49% <37.26%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
libvcx/src/api/filters.rs	63.15% <ø> (ø)
libvcx/src/api/logger.rs	11.11% <ø> (ø)
libvcx/src/api/proof.rs	52.73% <ø> (ø)
libvcx/src/api/return_types_u32.rs	64.06% <ø> (ø)
libvcx/src/aries/handlers/connection/agent_info.rs	58.09% <ø> (ø)
libvcx/src/aries/handlers/connection/connection.rs	66.40% <ø> (ø)
...aries/handlers/connection/invitee/state_machine.rs	71.78% <ø> (ø)
...ies/handlers/connection/invitee/states/complete.rs	86.00% <ø> (ø)
...ries/handlers/connection/invitee/states/invited.rs	55.55% <ø> (ø)
...c/aries/handlers/connection/invitee/states/null.rs	60.00% <ø> (ø)
... and 124 more

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[mirgee]

I like this 👍 , except for the conflicts I will have to go through - don't think it will be that bad, though