Suggestion: declare explicit support of ASCII-only charset at SDK level

[dj8yfo]

this line https://github.com/LedgerHQ/ledger-device-rust-sdk/blob/master/ledger_device_sdk/src/ui/gadgets.rs#L831 can be changed to

enum FieldError {
    NonASCIIChar, 
}

pub fn show(&self) -> Result<bool, FieldError> {
...

Probably two additional checks can be added after line https://github.com/LedgerHQ/ledger-device-rust-sdk/blob/master/ledger_device_sdk/src/ui/gadgets.rs#L693 to assert that name and value fields of Field are comprised only of ASCII chars (https://doc.rust-lang.org/std/primitive.str.html#method.is_ascii), and returning Err(FieldError::NonASCIIChar) otherwise.

It appears to me that reflecting the fact, that non-ascii charsets are not supported, with types is more robust than displaying randomly blank chunks without any indication to user, why this happens.

MultiFieldReview::show() can be unwrapped as well at the call sites, if ledger-device-rust-sdk user doesn't want to bother with handling errors,
or the known error variants can be matched and handled, and e.g. a widget displayed to user, stating that Error: non-ASCII payload is not supported and a corresponding APDU status returned, refusing to continue to process.