Fix various bugs in matrix graph

[ABorgna]

I made multiple changes and fixes to MatrixGraph:

1. The routine for extending the adjacency matrix for directed graphs created a new matrix and tried to copy the data doing a mem::swap of references to slices, which was effectively a no-op so the adjacencies were lost.
   I changed it to inplace by-element slice::swaps which surprisingly seem to be faster than what was there before. This fixes Edge disappearing, or misunderstanding matrix graph? #425.
   I tried a version that uses an unsafe ptr::swap_nonoverlapping when possible in ABorgna@877edbd and it had around 15% better performance in resize-heavy tests.

2. Many functions didn't check the adjacency matrix capacity before looking for a node data.
   This was masked in the tests due to new always reserving a 4x4 matrix.

3. I renamed the new capacity parameter in the extension functions to avoid confusions (was min_node).

4. I made with_capacity reserve the exact value passed instead of rounding to the next power of two, to avoid quadratic wastage.

Benchmarks:

 name                               base ns/iter  fixed ns/iter  diff ns/iter   diff %  speedup
 add_100_edges_to_self              426           282                    -144  -33.80%   x 1.51
 add_100_nodes                      19,950        12,841               -7,109  -35.63%   x 1.55
 add_5_edges_for_each_of_100_nodes  1,060         904                    -156  -14.72%   x 1.17
 add_adjacent_edges                 27,491        24,965               -2,526   -9.19%   x 1.10
 add_edges_from_root                27,372        25,196               -2,176   -7.95%   x 1.09
 bigger_edges_in                    35            36                        1    2.86%   x 0.97
 bigger_edges_out                   31            35                        4   12.90%   x 0.89
 bigger_kosaraju_sccs               5,272         5,083                  -189   -3.58%   x 1.04
 bigger_neighbors_in                36            36                        0    0.00%   x 1.00
 bigger_neighbors_out               32            35                        3    9.38%   x 0.91
 bigger_tarjan_sccs                 2,779         2,731                   -48   -1.73%   x 1.02
 full_edges_in                      14            15                        1    7.14%   x 0.93
 full_edges_out                     13            13                        0    0.00%   x 1.00
 full_kosaraju_sccs                 953           991                      38    3.99%   x 0.96
 full_neighbors_in                  14            15                        1    7.14%   x 0.93
 full_neighbors_out                 13            13                        0    0.00%   x 1.00
 full_tarjan_sccs                   435           442                       7    1.61%   x 0.98

[ABorgna]

Here is the benchmark diff for the unsafe copying vs the iterative version. I think I'll merge that change here.

 name                               fixed ns/iter  fixed_unsafe ns/iter  diff ns/iter   diff %  speedup
 add_100_edges_to_self              282            283                              1    0.35%   x 1.00
 add_100_nodes                      12,841         12,274                        -567   -4.42%   x 1.05
 add_5_edges_for_each_of_100_nodes  904            899                             -5   -0.55%   x 1.01
 add_adjacent_edges                 24,965         21,053                      -3,912  -15.67%   x 1.19
 add_edges_from_root                25,196         21,096                      -4,100  -16.27%   x 1.19
 bigger_edges_in                    36             37                               1    2.78%   x 0.97
 bigger_edges_out                   35             35                               0    0.00%   x 1.00
 bigger_kosaraju_sccs               5,083          5,049                          -34   -0.67%   x 1.01
 bigger_neighbors_in                36             36                               0    0.00%   x 1.00
 bigger_neighbors_out               35             35                               0    0.00%   x 1.00
 bigger_tarjan_sccs                 2,731          2,794                           63    2.31%   x 0.98
 full_edges_in                      15             15                               0    0.00%   x 1.00
 full_edges_out                     13             13                               0    0.00%   x 1.00
 full_kosaraju_sccs                 991            1,007                           16    1.61%   x 0.98
 full_neighbors_in                  15             15                               0    0.00%   x 1.00
 full_neighbors_out                 13             13                               0    0.00%   x 1.00
 full_tarjan_sccs                   442            441                             -1   -0.23%   x 1.00