Remove the argcAndIndex argument from instantiateJSToNativeConversionTemplate.

This commit does not change the generated code.
Remove the isOptional argument and the dealWithOptional return value from getJSToNativeConversionTemplate.

Note that dictionaries currently don't support members without default values
right now.

This commit does not change the generated code.
auto merge of #2394 : Ms2ger/servo/instantiateJSToNativeConversionTemplate, r=jdm

This moves all the handling of optional arguments into `CGArgumentConverter`, and leaves `instantiateJSToNativeConversionTemplate` to only deal with the declaration of stack variables.