Allow word count multiples of 3 instead of 6

BIP39 says

> The allowed size of ENT is 128-256 bits.

Once we add the checksum this is 132-264 bits. This is divided by 11 to
get the word count. From the BIP141
```
CS = ENT / 32
MS = (ENT + CS) / 11

|  ENT  | CS | ENT+CS |  MS  |
+-------+----+--------+------+
|  128  |  4 |   132  |  12  |
|  160  |  5 |   165  |  15  |
|  192  |  6 |   198  |  18  |
|  224  |  7 |   231  |  21  |
|  256  |  8 |   264  |  24  |
```

Currently we are limiting word count to be a multiple of 6, I do not
know why this is being done but from reading the BIP it seems that 15 is
a valid word count value.

Allow word count of 15 by checking the word count modulo 3 instead of
modulo 6. Since this check appears twice in the code, add a helper
function to reduce code duplication.

Fixes: #15

src/lib.rs

@@ -250,7 +250,7 @@ impl Mnemonic {
 	where
 		R: rand_core::RngCore + rand_core::CryptoRng,
 	{
-		if word_count < MIN_NB_WORDS || word_count % 6 != 0 || word_count > MAX_NB_WORDS {
+		if is_invalid_word_count(word_count) {
 			return Err(Error::BadWordCount(word_count));
 		}
 
@@ -377,7 +377,7 @@ impl Mnemonic {
 	/// Parse a mnemonic in normalized UTF8 in the given language.
 	pub fn parse_in_normalized(language: Language, s: &str) -> Result<Mnemonic, Error> {
 		let nb_words = s.split_whitespace().count();
-		if nb_words < MIN_NB_WORDS || nb_words % 6 != 0 || nb_words > MAX_NB_WORDS {
+		if is_invalid_word_count(nb_words) {
 			return Err(Error::BadWordCount(nb_words));
 		}
 
@@ -558,6 +558,10 @@ impl str::FromStr for Mnemonic {
 	}
 }
 
+fn is_invalid_word_count(word_count: usize) -> bool {
+	word_count < MIN_NB_WORDS || word_count % 3 != 0 || word_count > MAX_NB_WORDS
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;