Rename the PrototypeList enums to fix case build warnings.
Have 'mach build' pass arbitrary arguments to Cargo.

Duplicating all of Cargo’s argument parsing in mach is silly.
Remove usage of the deprecated std::num::Zero trait.
Remove usage of deprecated HashMap::get_copy
Simplify util::range::RangeIndex to always require std::num::Int,

and fix remaining warnings.
auto merge of #4420 : servo/servo/fix-warnings, r=Ms2ger