a number of cleanups to the design:
- no module imports in scripts
- default can only appear in a binding position as an `export function` or `export class` declaration name
- eliminated `export *;` form -- see https://mail.mozilla.org/pipermail/es-discuss/2013-November/034589.html
- can do combined anonymous/named import via `import $, { ajax } from "jquery"`
- eliminated `export x = 1` form
- added `export default AssignmentExpression;` form

also some internal spec cleanups:
- binding identifiers are BindingIdentifier instead of Identifier
- ImportNames is just a thin wrapper around BoundNames
- s/Specs/Entries/g
- s/ImportPairs/ImportSpecs/g
- s/ExportPairs/ExportSpecs/g
- eliminated ModuleKnownExports static semantics since `export *;` is gone
- BoundName checking for local exports is done in dynamic semantics (so we save M.[[BoundNames]])
- LookupModuleDependency can get a null requestName (and returns M)