Make parameter names more clear

lhs and rhs are confusing. I imagine they're especially confusing for
folks who read RTL languages.

Also, if you're using a `TypeConverter` to implement an
`IBindingTypeConverter`, the `from` and `to` distinctions helps you do
the right thing™.

BTW, if you like this change, I'm happy to change all the parameter names of the built-in converters.