Fix poor performance when one of batch's arguments is a Type{...}

[GianlucaFuwa]

This PR is a fix for #116 by wrapping each argument of batch in a wrap_type function and using unwrap_type before the start of the loop body, as @chriselrod suggested. A test is included in runtests.jl.

Also made it such that batch always returns an empty Tuple when reduction is not used, since it used to be a Union{Nothing, Tuple{}}.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.29%. Comparing base (c0ef9e1) to head (3038f82).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
- Coverage   94.66%   93.29%   -1.37%     
==========================================
  Files           3        3              
  Lines         600      582      -18     
==========================================
- Hits          568      543      -25     
- Misses         32       39       +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[chriselrod]

Thanks, this looks good to me!