Eliminate uses of SExpr from the parser. (#5496)

Eliminate uses of SExpr from the parser. (#5496)

This is a step towards migrating Commands to the API. SExpr is an internal module that's not used by the API but is used by the parser and some commands. This PR replaces SExpr with terms of kind SEXPR in the parser and strings in commands (which brings them more inline with the API).