Error messages can be unwieldy

[dsharlet]

When using constructors or operator() incorrectly, the error messages are often huge and hard to decipher, for a few reasons:

1. The template parameters of all of the shape parameters make symbol names huge.
2. The errors occur deep in the call stack in variadic templates, when the error should occur closer to the user's code.

Issue (2) could be improved with more aggressive use of static_assert earlier in the API.

Issue (1) is harder to address, but would be mitigated somewhat by addressing the second issue (because fewer symbols would be reported as part of errors).

[dsharlet]

I think this issue is now actually pretty good. Most/all rank mismatches are now reported as a single error at the call site, and the error is not hard to understand.

For example, using the wrong number of indices:

test/array_ref.cpp:39:5: error: no matching function for call to object of type 'dense_array_ref<int, 2>' (aka 'array_ref<int,
      nda::shape<nda::dim<-9, -9, 1>, nda::dim<-9, -9, -9> > >')
    ref_2d(x, y, 0) = 0;
    ^~~~~~
./array.h:1642:13: note: candidate template ignored: requirement 'sizeof...(Args) == rank()' was not satisfied [with Args = <int, int, int>]
  reference operator() (Args... indices) const { return base_[shape_(indices...)]; }
            ^
./array.h:1649:8: note: candidate template ignored: requirement 'sizeof...(Args) == rank()' was not satisfied [with Args = <int, int, int>]
  auto operator() (Args... ranges) const {
       ^
./array.h:1637:13: note: candidate function not viable: requires single argument 'indices', but 3 arguments were provided
  reference operator() (const index_type& indices) const { return base_[shape_(indices)]; }