Path and Identifier validation and improvements

[seanchen1991]

Addresses the remaining tasks in #14

• Replace basecoin-rs/src/app/store.rs:Identifier::is_valid() with ibc-rs/modules/src/ics24_host/validate.rs:validate_identifier().
• Deserialize paths into concrete ibc-rs/modules/src/ics24_host/path.rs:29. (Implement FromStr for Path hermes#1480)
• Does it make sense to use const-generics? e.g. (Although this will be stored on the stack, I think it increases memory usage because we're always reserving N chars. Moreover, I feel like this is something we should add to ibc-rs, so let's leave this for now.)

pub struct Identifier<const N: usize>([char; N]);