Move assignments outside match expressions in getJSToNativeConversionTemplate.

This is a first step towards making getJSToNativeConversionTemplate return an
expression, which will improve dictionary codegen in particular.
Move responsability for the assignment out of CastableObjectUnwrapper.

This is a second step towards making getJSToNativeConversionTemplate return an
expression, which will improve dictionary codegen in particular.

This commit does not make any change to the generated code.
Inline CGAbstractBindingMethod.getThis.

There's no good reason to keep this separate; it's only called once, and
inlining the function makes the code a little clearer.

This commit does not make any change to the generated code.
Assign to 'this' immediately in CGAbstractBindingMethod.

This makes the generated code a bit more rustic.
auto merge of #2227 : Ms2ger/servo/assign-outside-match, r=jdm