compiler internals: AST_STMT_FUNCTION is weird

[Akuli]

In the Jou compiler, an AstStatement with kind set to AST_STMT_FUNCTION works like this:

jou/src/jou_compiler.h

Lines 278 to 281 in 6171948

	struct AstFunction {
	AstSignature signature;
	AstBody body; // empty body means declaration, otherwise it's definition
	};

This is not a good idea. Maybe it was at some point, but now I only see this as a source of bugs.

Instead, declare and def statements should be separated clearly, e.g. AST_DECLARE_FUNC (declare statement) and AST_DEFINE_FUNC (def statement). The AstFunction struct would be renamed to something like AstFunctionDef and it would only be used for def. A declare can simply use an AstSignature.