Fix all v0.7 deprecations and errors, and convert tests to testsets

[rdeits]

This PR fixes all deprecation warnings and errors on v0.7-alpha and changes the minimum required Julia version to v0.7-. The only errors were caused by an ambiguity resulting from the Char -> AbstractChar change, which was easy to fix. Most of the remaining deprecation warnings were because of the removed fallback from constructors to convert(). In this case, I fixed them by explicitly calling convert where appropriate. You could, instead, define (::Type{T<:Number})(x::Fixed) = convert(T, x) to restore the missing constructors. I guess it's really just a matter of deciding what the "canonical" way of doing this is.

There were also a lot of depwarns in the tests themselves because a bunch of global variables were being accidentally overwritten in for-loops. I fixed that by adding labeled testsets, which puts each test in a local scope (which should reduce the chances of one test accidentally using variables from another) and also helps document the tests themselves. I guessed at the labels, but I think I mostly got them right.

[rdeits]

Alas, the github diff view really falls apart when you add a leading tab to every line in a file.... I'm going to remove the extra indentation so that the actual diff is more obvious.

[codecov-io]

Codecov Report

Merging #108 into master will increase coverage by 10.47%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #108       +/-   ##
=========================================
+ Coverage   89.52%   100%   +10.47%     
=========================================
  Files           4      3        -1     
  Lines         191    111       -80     
=========================================
- Hits          171    111       -60     
+ Misses         20      0       -20

Impacted Files	Coverage Δ
src/FixedPointNumbers.jl	100% <100%> (+15.71%)	⬆️
src/normed.jl	100% <100%> (+2.24%)	⬆️
src/fixed.jl	100% <100%> (ø)	⬆️

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[Keno]

julia 0.7-alpha.

[rdeits]

ok, done

[Keno]

Remove the deprecations while we're at it?

[rdeits]

Ok, I've removed the deprecations. The only thing we might still want to do is change some of the convert() methods to constructors (see https://discourse.julialang.org/t/recommended-style-for-conversion-vs-constructors-in-v0-7/11561 )

[Keno]

As per that discourse thread, looks like we need to use some more constructors (and maybe define them). Other than that, this LGTM.

[rdeits]

Ok, I changed all the existing convert() methods to constructors. As a bonus, removing the deprecations brings this package up to 100% test coverage!