fix(core): block FromStr implementation

[DaniPopes]

Motivation

Numbers get parsed first as hex, so a decimal number is default treated as hex

Solution

only parse as hex if prefixed by 0x
the prefix is handled by U64::from_str
only downside to current fix is that a non-prefixed hex gets parsed as decimal, ex: a is invalid while 0xa is valid

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog
• Breaking changes