Fix prototypes for Number, String and Boolean

[tofpie]

This Pull Request fixes #980.

It changes the following:

• Prototypes for String, Number and Boolean

[codecov]

Codecov Report

Merging #981 (964eaed) into master (d46080d) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #981      +/-   ##
==========================================
+ Coverage   59.56%   59.80%   +0.23%     
==========================================
  Files         166      166              
  Lines       10821    10842      +21     
==========================================
+ Hits         6446     6484      +38     
+ Misses       4375     4358      -17     

Impacted Files	Coverage Δ
boa/src/context.rs	64.46% <100.00%> (+3.20%)	⬆️
boa/src/object/mod.rs	51.41% <0.00%> (+4.95%)	⬆️

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