Naming fixes

[fjarri]

Fixes #33

• renames SecretKeyFactory::secret_key_by_label()/secret_factory_by_label() -> make_key()/make_factory(). The former names were way too wordy.
• replaces remaining (from Rename "num_kfrags" to "shares" #69) instances of m/n with threshold/shares.
• replaces verifying_key parameter in Signature::verify() with verifying_pk - it is called like that everywhere else.

[codecov-commenter]

Codecov Report

Merging #71 (d9327f3) into master (c0e03ef) will not change coverage.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   82.36%   82.36%           
=======================================
  Files          14       14           
  Lines        1083     1083           
=======================================
  Hits          892      892           
  Misses        191      191           

Impacted Files	Coverage Δ
umbral-pre/src/keys.rs	71.25% <85.71%> (ø)

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