runtime: improve some type annotations.
runtime: break out part of r5js.ProcCall#evalArgs into private method.
misc: begin cleaning up branch.js.
runtime: add TODO for implementing (values ...) in Scheme.
parse: remove two superfluous type casts.
runtime: improve type of param to r5js.Branch ctor.
runtime+misc: no need to create AST nodes for Branch ctor.
runtime+misc: strengthen more r5js.Branch ctor params.
runtime: remove r5js.Branch#debugString.
runtime: make r5js.Branch state private and const where possible.
runtime: add+use r5js.Branch#maybeSetEnv.
parse+runtime: move empty-alternate-branch construction to parser.
runtime: make r5js.Branch private and @struct.
runtime: run branch.js through the linter.
runtime: remove some debug code from r5js.trampoline.
runtime: move branch.js under runtime/.
runtime: start cleaning up r5js.Continuable.
runtime: make r5js.Continuable.subtype private and add a getter.
runtime+misc: make r5js.Continuable.continuation private, adding get+set.
runtime: add r5js.TopLevelAssignment subclass of Assignment.