add signature decoding

[squalus]

• add NarSignature decoding
• use cryptonite instead of saltine for signature
• remove saltine dependency
• add signature decode and verify tests taken from tvix
• start decoding signatures in queryPathInfoUncached

Closes #240 and #246

[sorki]

Good job, much appreciated!!

[sorki]

@Gabriella439 saltine is gone \o/ Also the dependency footprint of -core got significantly reduced in #247