Implement Object.prototype.isPrototypeOf

[tofpie]

This Pull Request closes #982 .

It changes the following:

• Implementation of `Object.prototype.isPrototypeOf
• Fix construct when the prototype of the constructor is not an object

[codecov]

Codecov Report

Merging #983 (f7c7c0b) into master (880792e) will increase coverage by 0.06%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #983      +/-   ##
==========================================
+ Coverage   59.67%   59.74%   +0.06%     
==========================================
  Files         166      166              
  Lines       10813    10830      +17     
==========================================
+ Hits         6453     6470      +17     
  Misses       4360     4360              

Impacted Files	Coverage Δ
boa/src/builtins/object/mod.rs	67.66% <69.23%> (+2.72%)	⬆️
boa/src/object/gcobject.rs	64.72% <100.00%> (+0.52%)	⬆️

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[Razican]

Looks good, thanks!

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,493	78,493	0
Passed	19,404	19,481	+77
Ignored	15,585	15,585	0
Failed	43,504	43,427	-77
Panics	447	448	+1
Conformance	24.72	24.82	+0.10%