Avoid additional heap allocation for fields

[dfordivam]

It may be possible to avoid additional allocation of the fields on heap by making use of APIs like core::ffi::CStr from_bytes_until_nul

ledger-device-rust-sdk/ledger_device_sdk/src/nbgl.rs

Lines 328 to 334 in 3c22c1c

	let v: Vec<CField> = fields
	.iter()
	.map(|f| CField {
	name: CString::new(f.name).unwrap(),
	value: CString::new(f.value).unwrap(),
	})
	.collect();

Also the unwrap may be avoided on user provided fields, and return an error instead.

[yogh333]

Ok but in that case, it means that you must work, at application level, with nul-terminated string with no nul bytes in the middle.

[dfordivam]

@yogh333 That is true even right now, unless the unwrap() is removed

[yogh333]

@dfordivam No, nowadays you can work with &str or String at application level and Rust SDK will convert them seamlessly into a C compatible string with CString::new. Avoiding additional heap allocation as you propose is thus counter balanced with less comfort at application level, as it implies adding systematically '\0' at the end of every string to be passed to NBGL object to be displayed.