monero: only mask user features on new polyseed, not on decode (#503)

* monero: only mask user features on new polyseed, not on decode

- This commit ensures a polyseed string that has unsupported features correctly errors on decode (rather than panic in debug build or return an incorrect successful response in prod build)
- Also avoids panicking when checksum calculation is unexpectedly wrong

Polyseed reference impl for feature masking:
- polyseed_create: b7c35bb3c6/src/polyseed.c (L61)
- polyseed_decode: b7c35bb3c6/src/polyseed.c (L212)

* PR comments

* Make from_internal a member of Polyseed

* Add accidentally removed newline

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>

coins/monero/src/tests/seed.rs

@@ -7,7 +7,7 @@ use curve25519_dalek::scalar::Scalar;
 use crate::{
   hash,
   wallet::seed::{
-    Seed, SeedType,
+    Seed, SeedType, SeedError,
     classic::{self, trim_by_lang},
     polyseed,
   },
@@ -469,3 +469,13 @@ fn test_polyseed() {
     }
   }
 }
+
+#[test]
+fn test_invalid_polyseed() {
+  // This seed includes unsupported features bits and should error on decode
+  let seed = "include domain claim resemble urban hire lunch bird \
+    crucial fire best wife ring warm ignore model"
+    .into();
+  let res = Seed::from_string(Zeroizing::new(seed));
+  assert_eq!(res, Err(SeedError::UnsupportedFeatures));
+}