Implement base64 encoder, avoid allocation in codecs_ecdsa_secp256k1.nim

[Vindaar]

As part of PR #490, which adds support for ECDSA we will add a basic PEM file writer for public and private keys. However, the current implementation depends on the standard library, because the data is base64 encoded. While we have a base64 decoder:

https://github.com/mratsim/constantine/blob/master/constantine/serialization/codecs.nim#L114-L174

we lack a base64 encoder.

Further, the current PEM file implementation allocates for the string we write to. These things should be addressed in the near future.

• Implement base64 encoder
• Write tests for base64 encoder
• Write tests for existing base64 decoder
• Avoid allocation for PEM file generator