Remove INVALID_BASE and use Option<NonZeroU32> for base

[vbkaisetsu]

I am not sure if the following part is correct, could you check carefully?

daachorse/src/charwise/builder.rs

Lines 324 to 327 in 4794bcc

	// len() is not 0 since states has at least block_len items.
	// The following value always larger than or equal to len() since block_len is
	// alphabet_size().next_power_of_two().
	NonZeroU32::new(u32::try_from(self.states.len()).unwrap() ^ edges[0].0).unwrap()

[kampersanda]

And, the following part is correct. Thank you for your modification.

daachorse/src/charwise/builder.rs

Lines 324 to 327 in 4794bcc

	// len() is not 0 since states has at least block_len items.
	// The following value always larger than or equal to len() since block_len is
	// alphabet_size().next_power_of_two().
	NonZeroU32::new(u32::try_from(self.states.len()).unwrap() ^ edges[0].0).unwrap()