this is a generalization of `check` vs. `assert` to other annotations. Not all
annotations are relevant though. Currently, there are more annotation nodes in
the AST that can have a flag `{ tp_only_check }` than was deemed useful in
pub/frama-c#25, but said flag can in fact safely be ignored. Moreover, this
commit only add this flag in the AST, but provides no further mean to set it to
true except for the original `check` keyword (i.e. on `AAssert`). The parser
and the behavior of the plugins that can handle the flag will be updated in
subsequent commits

These aliases will not be printed in the help message of Eva, and will emit
a warning when used.

[e-acsl] simplify and document Prepare_ast a bit more