Compile with warnings and fix exiting warnings

[dylanahsmith]

It is easy to make some stupid mistakes that wastes some time debugging when the compiler could have pointer out the problem. Using -Wall (with gcc/clang) will enable warnings for errors that compilers can detect.

I've addressed the existing warnings in the first few commits. I enabled compiler warnings by default when compiling locally. Additionally, I used -Werror to treat warnings as errors in CI.

I chose not to treat warnings as errors locally, mostly because of experience with lint like errors interrupting the development/debug feedback loop when they aren't really relevant. Although, this is kind of a philosophical divergence from Go (e.g. how it warns about unused imports when adding in import "fmt" for debugging), so I can enable -Werror locally if other v8go devs disagree.

[codecov]

Codecov Report

Merging #227 (612c439) into master (a582436) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   95.59%   95.59%           
=======================================
  Files          16       16           
  Lines         545      545           
=======================================
  Hits          521      521           
  Misses         15       15           
  Partials        9        9           

Impacted Files	Coverage Δ
object.go	100.00% <100.00%> (ø)
object_template.go	100.00% <100.00%> (ø)

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