[WIP] Enable automatic detecting and loading all filter/sketch types

[standage]

See #1716.

So far this PR replaces the ad hoc ht_type defines with an enumerated type, and adds values for the *table types. Now the question is how to distinguish between *graph and *table types in the storage class, which by design is agnostic to this.

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only backwards-compatible
  additions are allowed without a major version increment. Changing file
  formats also requires a major version number increment.
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)

[codecov-io]

Codecov Report

Merging #1717 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master   #1717   +/-   ##
======================================
  Coverage    0.05%   0.05%           
======================================
  Files          91      91           
  Lines       11483   11483           
  Branches     3056    3056           
======================================
  Hits            6       6           
  Misses      11477   11477

Impacted Files	Coverage Δ
src/khmer/_cpy_khmer.cc	0% <ø> (ø)	⬆️
src/oxli/storage.cc	0% <0%> (ø)	⬆️

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[standage]

It looks like we either need to store a ht_type in the storage object at creation time, or pass the value to storage when load and save are invoked. The latter seems less problematic, but neither option sounds very appealing.