comp: compile `lazy` in the getter
comp: compile `lazy` in the getter

For primitive and nullables, lazy is implemented with an additionnal
boolean attribute used as a guard.
The guard is also update in the setter.

For nonnullable val*, lazy is implemented with issset that is currently
only correctly implemented in this subcase.

One isset has a good implementation, the guard implemnatation can be
removed.

Signed-off-by: Jean Privat <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

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