BEL term completion using libbel AST

[abargnesi]

This library includes completion on BEL expressions (see BEL::Completion).

This work would be to use the included C extension to parse and provide completions from the Abstract Syntax Tree.

Goals

• Leverage bel_parse_term to produce BEL term AST. We can provide a BEL term-based completion now and eventually extend this to a BEL statement once we enhance the AST parser.
• Improve provided completions. For example the AST can recognize a namespace value so it is logical to provide fully constructed BEL term completions based on the encoding values (e.g. ProteinConcept, RNAConcept, etc.).