Parse genesis account amount as string

[JamesHinshelwood]

Annoyingly toml (the crate) doesn't support u128 (toml-rs/toml#540), so we instead take the configured values as strings and parse them into u128s later.

[theo-zil]

Whoops, I should've checked this PR first before commenting about this on the other one.

I wonder if it would be ergonomic to use a custom deserializer function that uses deserialize_string() from serde and then .parse()s it into a u128 in-place? The benefit being proper type-safety in our Config type, where a malformed string would fail on file parsing, rather than happily progressing through to application logic with a malformed config and then being liable to fail on a parse() call way down in the State constructor.

But pragmatically speaking we will always construct a State very early when starting the node so it'll still fail fast, so if this sounds like a hassle then I've no major qualms with merging as-is.

EDIT: Didn't notice auto-merge was on, haha. Fair enough, sounds like we're doing it this way then, if you think it'd be worthwhile to come back to my suggestion then we can open an issue for later, since like I said pragmatically speaking this is fine as far as I'm concerned.