error+runtime+test: improve handling of r5js.UnimplementedOptionError.
runtime: allow (display <value> <custom-output-port>).
ast: add r5js.DatumType.BOOLEAN, which apparently wasn't needed until now.
runtime: add r5js.PrimitiveProcedures.getActualType_ for error pretty-printing.
runtime+error: improve r5js.ArgumentTypeError.
test: improve r5js.ArgumentTypeError test coverage.
ast: remove no-longer-used r5js.DatumType.FFI.
ast: rename r5js.DatumType.VECTOR value from "#(" to "vector".
ast: add r5js.DatumType.PROCEDURE.
error+test: improve r5js.TooFewVarargs test coverage.
error+test: improve r5js.TooManyVarargs test coverage.