Add purser-core PurserSigner

[rdig]

This PR adds a "Signer" to the purser-core module, extending and implementing the required class methods, as specified by the Ethers Signer.

This branch also:

• gets rid of yarn and switches all scrips that are run to npm in order to get the repo more in line with the rest of JoinColony's repositories.
• gets rid of the Circle CI build, and replaces it with a Github Actions backed build workflow

Added:

• puser-core PuserSigner class
• PuserSigner unit tests
• PuserSigner documentation