Removed Compat/Missings and bring code up to Julia 1.x

[tk3369]

This is needed to register v1.0.0.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@dc7249a). Click here to learn what that means.
The diff coverage is 75%.

@@            Coverage Diff            @@
##             master      #60   +/-   ##
=========================================
  Coverage          ?   92.42%           
=========================================
  Files             ?        8           
  Lines             ?      779           
  Branches          ?        0           
=========================================
  Hits              ?      720           
  Misses            ?       59           
  Partials          ?        0

Impacted Files	Coverage Δ
src/constants.jl	100% <100%> (ø)
src/Metadata.jl	100% <100%> (ø)
src/SASLib.jl	92.01% <70%> (ø)

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