Expect shape

[MichaelChirico]

Closes #1423

Implementation as of now has a few design choices, happy to adapt as seen fit:

• Added the function to R/expect-length.R since it fits so nicely alongside expect_length(), but gave it its own test file.
• Subsumes expect_length(), i.e., expect_shape(x, n) matches expect_length(x, n). Makes it easier for a series of tests to all use expect_shape(), instead of jumping between expect_length() and expect_shape() from test to test
• Allows expect_shape(., nrow=m, ncol=n), which is usually the same as expect_shape(., c(m, n)), expect when length(dim(.)) > 2, a rather boutique use case of wanting to test only the first 2 dimensions of an array.

[hadley]

By convention, we now match tests/testthat/test-abc.R to R/abc.R.

[MichaelChirico]

Sounds good... would you prefer expect_shape in its own file, or these tests moved over to test-expect-length?