Add predicates package and über-import for all predicates

[fthomas]

This allows to import all predicates with import predicates.all._. The AllPredicates trait also allow us to create an even bigger über-import. It is one step towards #184.

With a predicates package it also makes sense to move the actual definitions of our predicates here. But that should happen in another PR.

[codecov]

Codecov Report

Merging #433 into master will decrease coverage by 1.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   98.84%   97.56%   -1.28%     
==========================================
  Files          51       57       +6     
  Lines         605      658      +53     
  Branches       12       12              
==========================================
+ Hits          598      642      +44     
- Misses          7       16       +9

Impacted Files	Coverage Δ
.../scala/eu/timepit/refined/predicates/numeric.scala	100% <100%> (ø)
...ala/eu/timepit/refined/predicates/collection.scala	100% <100%> (ø)
.../scala/eu/timepit/refined/predicates/boolean.scala	100% <100%> (ø)
.../scala/eu/timepit/refined/predicates/generic.scala	100% <100%> (ø)
...ain/scala/eu/timepit/refined/predicates/char.scala	100% <100%> (ø)
...n/scala/eu/timepit/refined/predicates/string.scala	100% <100%> (ø)
... and 2 more

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