Add functions for comparing type-level Nats and Symbols.

(cherry picked from commit c1d3546420ee482bbbd9f15d45a6e8a26304d419)
Comments only.

Fix the documentation on CmpSymbol and CmpNat; add a note on (<=?).

(cherry picked from commit 5edb063688e73ec00fd1f61ac0e8317dd122f44a)
add /Since/ annotation

(cherry picked from commit 090ca42d43578310fe033df0a21134370dc6ddc3)