Standard library: Base64

Base 64 encoding and decoding.

Existing source:

[dafny-lang/libraries/pull/3](https://github.com/dafny-lang/libraries/pull/3)
[aws/aws-cryptographic-material-providers-library-dafny/StandardLibrary/src/Base64.dfy](https://github.com/aws/aws-cryptographic-material-providers-library-dafny/blob/main/StandardLibrary/src/Base64.dfy)