Feat: Rename Object::prototype() and Object::set_prototype()

[RageKnify]

This Pull Request fixes/closes #607.

It changes the following:

• Rename Object::prototype() to Object::prototype_instance()
• Rename Object::set_prototype() to Object::set_prototype_instance()

[codecov]

Codecov Report

Merging #693 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
- Coverage   71.70%   71.69%   -0.02%     
==========================================
  Files         199      199              
  Lines       13863    13865       +2     
==========================================
  Hits         9941     9941              
- Misses       3922     3924       +2     

Impacted Files	Coverage Δ
boa/src/builtins/json/tests.rs	100.00% <ø> (ø)
boa/src/builtins/object/mod.rs	51.19% <0.00%> (+0.60%)	⬆️
boa/src/exec/tests.rs	100.00% <ø> (ø)
boa/src/value/display.rs	86.36% <ø> (ø)
boa/src/context.rs	60.79% <33.33%> (-1.06%)	⬇️
boa/src/builtins/array/mod.rs	76.65% <50.00%> (ø)
boa/src/builtins/boolean/tests.rs	100.00% <100.00%> (ø)
boa/src/builtins/map/mod.rs	82.25% <100.00%> (ø)
boa/src/object/mod.rs	25.97% <100.00%> (ø)
boa/src/value/mod.rs	69.68% <100.00%> (ø)

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[RageKnify]

I don't know why the benchmark check failed.
But I've found the reason for the bad coverage results, as mentioned in xd009642/tarpaulin#351, tarpaulin mistakenly considers chained method calls as not hit if split among multiple lines, resulting in situations like the following where lines 337-339 and 341-343 are considered as not being covered by the tests:

In this case cargo fmt increased the number of lines because of the new longer method name and it increased the number of lines considered to not be covered.

[HalidOdat]

I don't know why the benchmark check failed.

The benchmarks are failing because of an unrelated issue (critcmp is not compatible with criterion v0.3.3 BurntSushi/critcmp#6).