App size

[neithanmo]

This PR aims to reduce the app size, applying some techniques that were used with another Rust app(AVAX):

• optimize for size using "z" flag.
• Remove Debug symbols.
• Avoid panicking replacing as much indexing as possible, with get(), get_mut variants.
• Update zxlib to the latest version which includes checks for character sizes in nanos, this fixes #142

:link: zboto Link

[ftheirs]

Closing this https://github.com/Zondax/ledger-stacks/issues/145