feat: support key/export and key/import

[richardschneider]

Needs to update dependency on PR ipfs-inactive/interface-js-ipfs-core#187

[richardschneider]

@diasdavid Two tests are failing in .miscellaneous. This has nothing to do with my changes.

.miscellaneous
       "before all" hook:
     Error: Uncaught AssertionError: expected 'timeout' to not exist (test/interface/generic.spec.js:27486)

  2) .miscellaneous
       "after all" hook:
     Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

[codecov]

Codecov Report

Merging #653 into master will decrease coverage by 0.13%.
The diff coverage is 60%.

@@            Coverage Diff            @@
##           master    #653      +/-   ##
=========================================
- Coverage   84.54%   84.4%   -0.14%     
=========================================
  Files         122     124       +2     
  Lines        1786    1796      +10     
=========================================
+ Hits         1510    1516       +6     
- Misses        276     280       +4

Impacted Files	Coverage Δ
src/key/index.js	100% <ø> (ø)	⬆️
src/key/export.js	50% <50%> (ø)
src/key/import.js	75% <75%> (ø)

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