api: make updating a key safer

[mathstuf]

A key must be of a certain type, so request updating to respect that
decision. Also add a function to ask whether a key is of a given
keytype.

Fixes: #35

[codecov]

Codecov Report

Merging #45 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   86.65%   86.69%   +0.04%     
==========================================
  Files          34       35       +1     
  Lines        2105     2119      +14     
==========================================
+ Hits         1824     1837      +13     
- Misses        281      282       +1     

Impacted Files	Coverage Δ
src/api.rs	67.78% <100.00%> (-0.02%)	⬇️
src/tests/keytype.rs	100.00% <100.00%> (ø)
src/tests/update.rs	100.00% <100.00%> (ø)

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