Make a bunch of instances traits/classes private/sealed

[ceedubs]

This is related to #496, but it's a broader issue than just this change.

These instances traits/classes are used internally for code organization
and implicit prioritization, but they aren't really meant to be part of
the public API. Making them private/sealed may be helpful for binary
compatibility in the future. It also may be a helpful indicator to
newcomers which traits/classes are actually meant for consumption, so
they can ignore some of the noise.

So far, I have left the traits for orphans from std (such as
StringInstances), and I have left the omni-instance traits exposed.

[codecov-io]

Current coverage is 76.29%

Merging #612 into master will not affect coverage as of e300ab1

@@            master    #612   diff @@
======================================
  Files          160     160       
  Stmts         2194    2194       
  Branches        68      68       
  Methods          0       0       
======================================
  Hit           1674    1674       
  Partial          0       0       
  Missed         520     520       

Review entire Coverage Diff as of e300ab1

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[adelbertc]

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[mpilquist]

👍