Support structs in JS

[samestep]

Similar to #33, we need to translate JavaScript record types to Rose tuples. In this case we definitely should do the Proxy thing though, because the field names are not going to be Rose-computed values. One question is whether we should force the user to specify the struct type name when constructing a value directly, or whether they should just be able to use unadorned object literal notation; one possible downside of the latter is that it may result in the Rose JS frontend constructing more unnecessary tuple typedefs under the hood.