Generate API for pallet constants

[gregdhill]

#207 introduced a getter for PalletMetadata but as the strings are not validated at compile-time they may eventually fall out-of-sync. It would be better it add a ConstantsApi in the codegen crate.

[jsdw]

I wonder how much work it would be to avoid needing runtime metadata at all, since in theory everything metadata related should be doable at compile time. It's something that's crossed my mind a bit anyway, but I haven't yet looked into it!

[gregdhill]

We'll likely still need runtime metadata to validate the current API (#398).