add initial assertion library

[MKrupauskas]

Adds initial test assertion library. Closes #147 and more context in the issue.

This PR just introduces the library to make reviews easier. It will be used in another PR after this.

Unit tested.

$ make unittest
=== RUN   TestEqual
=== RUN   TestEqual/equal
=== RUN   TestEqual/not_equal_shallow
=== RUN   TestEqual/not_equal_deep
=== RUN   TestEqual/with_message
=== RUN   TestEqual/with_message_and_args
=== RUN   TestEqual/only_args
--- PASS: TestEqual (0.00s)
    --- PASS: TestEqual/equal (0.00s)
    --- PASS: TestEqual/not_equal_shallow (0.00s)
    --- PASS: TestEqual/not_equal_deep (0.00s)
    --- PASS: TestEqual/with_message (0.00s)
    --- PASS: TestEqual/with_message_and_args (0.00s)
    --- PASS: TestEqual/only_args (0.00s)
=== RUN   TestNoError
=== RUN   TestNoError/no_error
=== RUN   TestNoError/with_error
=== RUN   TestNoError/with_message
=== RUN   TestNoError/with_message_and_args
=== RUN   TestNoError/only_args
--- PASS: TestNoError (0.00s)
    --- PASS: TestNoError/no_error (0.00s)
    --- PASS: TestNoError/with_error (0.00s)
    --- PASS: TestNoError/with_message (0.00s)
    --- PASS: TestNoError/with_message_and_args (0.00s)
    --- PASS: TestNoError/only_args (0.00s)
PASS
coverage: 100.0% of statements
ok      github.com/go-zookeeper/zk/internal/assert      1.371s  coverage: 100.0% of statements

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.71%. Comparing base (27bc0d6) to head (30e5002).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   79.12%   79.71%   +0.59%     
==========================================
  Files           8        9       +1     
  Lines        1365     1385      +20     
==========================================
+ Hits         1080     1104      +24     
+ Misses        196      193       -3     
+ Partials       89       88       -1     

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