C#: Fix various extraction errors

[calumgrant]

Fix 3 classes of extraction error:

1. Extracting multi-dimensional arrays, e.g. byte *[][] tripped up the typemention logic.
2. Not treating _ as a regular variable name in a foreach, resulting in an unextracted local variable.
3. nameof(System), where System is a namespace resulted in an unknown identifier type.

The last one required DB changes, because a NameOfExpr should ideally have a getAccess(), which in turn means that Namespace should be @accessible and we needed a new type of expression, @namespace_access_expr to represent this access. This is not super important but may as well be done for completeness.

[hvitved]

The unit tests are failing, I will have a look once the PR is green.

[hvitved]

Two questions, otherwise LGTM.

[calumgrant]

Dist-compare results are as follows:

Project              fc0ffedda7a@1c51c3468  fc0ffedda7a@84fa0fb6d
corefx               4011                   3750                   0.934929
OrchardCore          2062                   2026                   0.982541
mono                 17798                  17739                  0.996685
OneOf                1541                   1537                   0.997404
PowerShell           1579                   1580                   1.00063
ql                   581                    584                    1.00516
EntityFrameworkCore  3464                   3496                   1.00924
coreclr              913                    929                    1.01752
roslyn               4403                   4487                   1.01908