Roll default_value into description for ConstraintSyntax
Remove unused struct ConstraintSetSyntax
Rename ParseTreeCheckFn -> PromiseCheckFn, update comment
Introduce enum SyntaxStatus for ConstraintSyntax, BodyTypeSyntax, PromiseTypeSyntax

Set all current syntax elements to SYNTAX_STATUS_NORMAL
Add parser warnings for deprecated or removed constraints

Set output constraints as SYNTAX_STATUS_REMOVED
Add SyntaxStatus to parser for body selections
Add SyntaxStatus to promise types in parser.

Set outputs promise type as SYNTAX_STATUS_REMOVED
Add StringStartsWith
Add -W (warnings) option to cf-promises, similar to gcc
Merge pull request #578 from sigurdteigen/feature/syntax-warnings

GCC-style warnings for cf-promises