refactor(builtin/Nat): rename destruct to discriminate

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
style(builtin/Nat): name convention

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(lean): add tutorial draft, and fix lexical documentation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>