pbkdf2: remove default simple feature

[tarcieri]

I suspect that most users of this crate care about the low-level pbkdf2 API as opposed to the higher-level password hashing API which implements the PHC string format.

The simple API pulls in quite a few dependencies (to the point I'm encountering dependency conflicts).

[tarcieri]

Here's an example resolution failure I'm encountering via a transitive dependency:

error: failed to select a version for `base64ct`.
    ... required by package `pem-rfc7468 v0.6.0`
    ... which satisfies dependency `pem-rfc7468 = "^0.6"` of package `der v0.6.0`
    ... which satisfies dependency `der = "^0.6"` of package `ecdsa v0.14.4`
    ... which satisfies dependency `ecdsa = "^0.14"` of package `signatory v0.26.0`
    ... which satisfies dependency `signatory = "^0.26"` of package `iqkms-signing v0.0.0 (/Users/bascule/src/iqlusion/iqkms/iqkms-signing)`
    ... which satisfies path dependency `signing` (locked to 0.0.0) of package `iqkms-ethereum v0.0.0 (/Users/bascule/src/iqlusion/iqkms/iqkms-ethereum)`
    ... which satisfies path dependency `ethereum` (locked to 0.0.0) of package `iqkmsd v0.0.0 (/Users/bascule/src/iqlusion/iqkms/iqkmsd)`
versions that meet the requirements `^1.4` are: 1.5.1, 1.5.2, 1.5.0, 1.4.1

all possible versions conflict with previously selected packages.

  previously selected package `base64ct v1.0.1`
    ... which satisfies dependency `base64ct = ">=1, <1.1.0"` of package `password-hash v0.3.2`
    ... which satisfies dependency `password-hash = "^0.3"` of package `pbkdf2 v0.10.0`
    ... which satisfies dependency `pbkdf2 = "^0.10.0"` of package `eth-keystore v0.4.1`

[tarcieri]

In addition to this change, I think we're about due for another release of password-hash

[newpavlov]

Can you also update changelog for pbkdf2? Also, I think it could be worth to include some ideas mentioned in #335. I may create a PR for it a bit later.

[tarcieri]

Sure. I'm in no rush to get another release out, so I think exploring some of the ideas from #335 would be great.