Add symbol manager (#5380)

Add symbol manager (#5380)

This add the symbol manager class, which is a Term-level utility, separate of the API. This class manages things like expression and assertion names, which is intentionally done outside the solver.

The symbol manager is intended to live at the same level as the Solver. When parsing input, the symbol manager will be used to model any interaction of e.g. named expressions and assertions. It also stores the symbol table of the parser.

This PR adds the basic interface for the symbol manager and passes it to the parser.

Later PRs will migrate the functionality for named expression out of e.g. SmtEngine and into SymbolManager. Commands will take Solver+SymbolManager instead of Solver. This will allow the parser to be fully migrated to the new API.

Marking "complex" since this impacts further design of the parser and the code that lives in src/main.

FYI @4tXJ7f