Trait insertions allowed and shrinkable corrections

[balmas]

From Javier Quinteros:

• In "Trait: Insertions allowed”: both operations can be replaced by “Insert at (index)”
• In “Trait: Shrinkable”: property “no deletes” would lead to a double negative expression. If needed, replace by "deletes-allowed” or something similar. But why not to use the “read-only” property?

[balmas]

to be addressed in #16