Enable bool type as storage type

[tqchen]

Previously, the bool type(uint1) cannot be directly stored and have to be explicitly cast to int8 before we store. However, it is still useful to introduce the type bool as it is used extensively in numpy and other languages. This PR enables this

• bool is stored as int8
• storage_flatten pass and buffer is updated to specially handle bool, and do type cast after loading, and before we store.
• The frontend will print uint1 type directly as bool, so the type flag is numpy compatible

[tqchen]

cc @jroesch @MarisaKirisame

[tqchen]

This likely also requires us to make followup changes to the relay constant expression to take care of the fact that bool type is stored as int8, while its type fields will read as uint(1)

[MarisaKirisame]

In DCE it is done as such. I think we can move the is_literal function into a common helper if anyone else need it.

[MarisaKirisame]

LGTM