compiler: introduce `AAttrPropdef::evaluate_expr` to factorize the compilation of default values

Signed-off-by: Jean Privat <jean@pryen.org>
tools: accept statement block in attributes

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_attr_init_val_block.nit

Signed-off-by: Jean Privat <jean@pryen.org>