ht: Add descriptive text representations

This patch adds text descriptions to all T* checks. Converting to a
string (e.g. str(ht.TNone)) will produce a user-readable text. This
will be used for Opcode parameter documentation.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>