Microptimize some buffer protocol and vector funcs

[Skylion007]

• Noticed some unnecessary copies in the templated lambdas, pytype objects aren't trivially copyable so it's better to pass them by reference here.

[mosra]

Thank you!

[codecov]

Codecov Report

Merging #15 (8809a25) into master (0510bdf) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   91.24%   91.24%           
=======================================
  Files          43       43           
  Lines        4535     4535           
=======================================
  Hits         4138     4138           
  Misses        397      397           

Impacted Files	Coverage Δ
src/python/corrade/containers.cpp	85.71% <100.00%> (ø)
src/python/magnum/math.matrix.h	93.22% <100.00%> (ø)
src/python/magnum/math.vector.h	98.58% <100.00%> (ø)

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[mosra]

Merged as ca5e16f, thanks!