cashc compiler

• ✨ Add .slice(start, end) operator for bytes and strings.
• ✨ Add bounded bytes typing and bounds checking for .split() (includes checking for negative indices).
• 🐎 Add optimisation for .slice(0, x) and .slice(x, y.length) (also applies to .split(0)[1]).
• 🐛 Disallow incorrect bounded bytes typing when using .split().

CashScript SDK

• 🐛 Fix bug with where ElectrumNetworkProvider would disconnect in browser on visibility change of the page.