fix(matrix): respect the documented bounds in set_slice, slice and transpose

[tachsin]

Fixes #844.

Four places where Matrix panicked on inputs its documentation covers.

• set_slice is documented to copy only the cells that fit when the slice goes outside the matrix, but a position past the edge underflowed self.rows - row and panicked. It now uses saturating_sub, and copies nothing when nothing fits.
• slice already returns WrongIndex for a range running past the matrix. A range ending before it starts underflowed rows.end - rows.start and panicked with a capacity overflow; it now gets WrongIndex too, and the documentation says so.
• transpose, in place, now refuses a matrix without rows with exactly the check and message transposed already uses, and documents it under # Panics. Before, debug builds underflowed and release builds left a matrix with rows of zero columns, which Matrix otherwise never allows. Nothing inside the crate calls transpose; the internal callers use transposed, which already had this check.
• utils::constrain, behind Matrix::constrain and Grid::constrain, negated negative values, and -isize::MIN does not fit in an isize. It now uses unsigned_abs, which gives the same result for every other value.

Tests

In tests/matrix.rs and tests/utils.rs:

• set_slice_outside_the_matrix_changes_nothing, including a position partly inside to check that clipping still copies what fits
• slice_with_a_reversed_range_is_an_error, for both rows and columns
• transpose_in_place_refuses_what_transposed_refuses, matching the panic message
• constrain_the_most_negative_value, against rem_euclid

Each fails on current main; the constrain one in a debug build, since release happened to wrap to the right answer.

For the rest of Matrix I compared rotations in both directions for every count, both flips, transposed and the in-place transpose (including the non-square cycle-following path), slice, neighbours, in_direction and constrain against straightforward reimplementations on a few hundred random matrices up to 6x6. They all agreed, so these fixes are limited to the edges listed above.

[samueltardieu]

Good catch