Remove ReadOnlyDictionary safety wrappers

[SergeiPavlov]

Also:

• Replace Pair<FieldInfo> by value tuple for readability

[botinko]

As far as I understand, the wrapper's overhead is quite small. I afraid that these changes may make future support of our fork more difficult for us, as changes from the DO master will increase.

[SergeiPavlov]

As far as I understand, the wrapper's overhead is quite small. I afraid that these changes may make future support of our fork more difficult for us, as changes from the DO master will increase.

They are trivial to resolve. The upstream branch changes very little last years.
There were many wrappers like this, and together the add some performance tax for us -may be small, may be not, it is better to be sure we don't pay whatever we need not