Use constants and not strings for signaling algorithms

SoftHSM currently use strings when signaling what algorithm to use. This is ineffective and require a lot of transformations and comparisons when doing a single crypto operation.

Suggesting to change this into constants. Should it use the ones defined in PKCS#11 (e.g. CKM_SHA256) or define our own internal values?

The crypto code is currently independent of the PKCS#11 definitions.