idl: slightly change `str` and `repr` for AST types
idl: slightly change `str` and `repr` for AST types

Surround string representation with angle brackets. This improves
readability when printing debug output.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla