Clean library

• Closes #11 (closed).
• Closes #26 (closed) (through https://gitlab.science.ru.nl/cstaps/clean-tools), though cpm has no support yet.

To do:

• Library for serialization/deserialization
• File I/O for serialized expressions
• Patch clm to optimise ABC and generate and link bytecode
• Implement a bunch of instructions that are used in the new library
• In string-to-graph, correctly create split HNF nodes
• Fix all tests in GraphTest
• Add examples to CI
• Check TODOs added to the codebase

Follow-ups:

• #58 (closed)
• #35 (closed)
• #56 (closed)