Enable `ink_metadata` to be used for metadata generation outside of `ink!`

[xermicus]

In order to fix this issue together with this change, we want to enable metadata construction at runtime. In Solang, we'd like to rely on the official ink! crates instead of maintaining our own solution. Additionally, it will be useful for anyone attempting to produce metadata for substrate contracts outside cargo-contract (e.g. when other languages want to add a Substrate target).

So far, this PR:

• Adds new constructors where needed
• Opens up some builder to be generic over Form
• Makes String the default String type

Ideally this should not break any existing API.