Path and Identifier validation and improvements

[hu55a1n1]

Description

• We must also define concretely what an Identifier is and validate it in accordance with ICS024.
• Path should ideally be a trait. Must provide methods for splitting into component Identifiers, append, etc.
• Replace basecoin-rs/src/app/store.rs:Identifier::is_valid() with ibc-rs/modules/src/ics24_host/validate.rs:validate_identifier().
• Reuse ibc-rs/modules/src/ics24_host/identifier.rs:{ChainId, ClientId, *} in basecoin-rs/src/app/modules/ibc.rs.
• Deserialize paths into concrete ibc-rs/modules/src/ics24_host/path.rs:29.
• Does it make sense to use const-generics? e.g.

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

[hu55a1n1]

Regarding Path as a trait, I think it's more intuitive to have a concrete type (i.e. store::Path) that is used as a key for the KV store. Each module may then define its own Path type (expected to be an enum) with all possible paths accessed by the module represented as enum variants. Each module may additionally define conversion methods to/from store::Path.