feat: publicize CommittableColumn (#189)

# Rationale for this change

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This change allows for custom column representations to be used in the
commitment API by implementing a borrowing conversion to
`CommittableColumn`. For my use case, I preferred doing this over
implementing a conversion to `OwnedColumn` to avoid copying, and over
implementing a conversion to `Column` to avoid invoking an allocator.

# What changes are included in this PR?

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`CommittableColumn` is made public.

# Are these changes tested?

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These changes do not affect functionality, which is verified by existing
tests.

crates/proof-of-sql/src/base/commitment/mod.rs

@@ -9,7 +9,7 @@ use core::ops::{AddAssign, SubAssign};
 use curve25519_dalek::ristretto::RistrettoPoint;
 
 mod committable_column;
-pub(crate) use committable_column::CommittableColumn;
+pub use committable_column::CommittableColumn;
 
 mod vec_commitment_ext;
 pub use vec_commitment_ext::{NumColumnsMismatch, VecCommitmentExt};