Fix Parallelism Cost when NumCPU() > 255

[josephlr]

As pointed out in #363, we are not currently handling cases with 256 or more CPUs correctly. Our current behavior:

• Casts costs.parallelism to a uint8
• Panics if NumCPU is a multiple of 256
• If costs.parallelism is > 256, we silently use the truncated parallelism cost. This is the worst part.

This PR does multiple things (see the commits for more detailed information):

• Increases cost error checking before calling into the Argon2 library.
• Adds a truncation_fixed boolean field to the HashingCosts proto, indicating that this bug is fixed.
• In getHashingCosts, cap the number of threads to 255.

[ebiggers]

Thanks for doing this! Two comments:

• Bug: at the bottom of getHashingCosts(), a HashingCosts is being initialized without TruncationFixed: true.
• func (h *HashingCosts) CheckValidity() already does some of the new checks that are being added to PassphraseHash(). Should they be consolidated into one place?