Added compile time checks to force swift-crypto api build in CryptoEx…

Incorrect maximum encryption size for RSA OAEP (#423)

Incorrect maximum encryption size for RSA OAEP

### Checklist
- [X] I've run tests to see all new and existing tests pass
- [X] I've followed the code style of the rest of the project
- [X] I've read the [Contribution Guidelines](CONTRIBUTING.md)
- [X] I've updated the documentation if necessary

#### If you've made changes to `gyb` files
- [n/a] I've run `./scripts/generate_boilerplate_files_with_gyb.sh` and
included updated generated files in a commit of this pull request

### Motivation:
The `maximumEncryptSize` function is hardcoded to use 42 as the hash
offset, but the RFC actually says it's "2*hLen-2" so 42 is only valid
for SHA1. SHA256 should be 62 (2*32-2). This adds a hash length onto the
Digest enum, which can then be used in the length calculation.

In writing the tests for this, I also stumbled on the unsafe PEM
representation for RSA Public Keys don't allow 1024-bit keys as
documented, so this also fixes that.

### Modifications:
* Added a `hashBitLength` to the RSA Digest which is then used in the
`maximumEncryptSize` to properly compute the maximum length
* Corrected the minimum key size for RSA unsafe PEM public keys to 1024
from 2048

### Result:
* The `maximumEncryptSize` function will return the expected value for
RSA OAEP SHA256 keys. I don't believe anything calls this internally, so
this would be for external consumers.
* Unsafe construction of RSA public keys now allow 1024 bit keys. It was
documented as supporting them, but the check was still 2048 (probably
copy-paste error from the safe variant). This only expands the potential
uses, so it shouldn't introduce any new failures.

Co-authored-by: Mike Schrag <[email protected]>