Replace IntervalTree with Vec

[philipc]

IntervalTree doesn't perform as well as a simple tree of Vecs. Note that a Vec tree can't handle partially overlapping intervals, but we don't need that.

 name                                        before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 context_new_and_query_location_rc           2,427,320       2,432,182             4,862    0.20%   x 1.00
 context_new_and_query_location_slice        765,873         758,006              -7,867   -1.03%   x 1.01
 context_new_and_query_with_functions_rc     2,993,102       2,959,278           -33,824   -1.13%   x 1.01
 context_new_and_query_with_functions_slice  1,133,112       1,152,324            19,212    1.70%   x 0.98
 context_new_parse_functions_rc              31,235,025      33,387,535        2,152,510    6.89%   x 0.94
 context_new_parse_functions_slice           23,754,161      26,858,657        3,104,496   13.07%   x 0.88
 context_new_parse_lines_rc                  11,725,686      11,711,061          -14,625   -0.12%   x 1.00
 context_new_parse_lines_slice               7,337,852       7,283,802           -54,050   -0.74%   x 1.01
 context_new_rc                              2,207,283       2,162,930           -44,353   -2.01%   x 1.02
 context_new_slice                           613,189         603,892              -9,297   -1.52%   x 1.02
 context_query_location_rc                   728,349         733,083               4,734    0.65%   x 0.99
 context_query_location_slice                721,907         729,294               7,387    1.02%   x 0.99
 context_query_with_functions_rc             4,292,707       1,952,886        -2,339,821  -54.51%   x 2.20
 context_query_with_functions_slice          4,302,388       1,744,765        -2,557,623  -59.45%   x 2.47
 new                                         28,964          27,013               -1,951   -6.74%   x 1.07
 new_unresolved_and_resolve_separate         29,332          26,986               -2,346   -8.00%   x 1.09
 trace_and_resolve_callback                  13,379          11,471               -1,908  -14.26%   x 1.17
 trace_and_resolve_separate                  11,013          9,801                -1,212  -11.01%   x 1.12

For the test parse_functions_slice, memory usage increased from 17.4MB to 19.7MB.

Also save some memory by using Box for line rows.

[coveralls]

Coverage increased (+6.4%) to 76.95% when pulling 8c79143 on philipc:no-interval-tree into 93163d2 on gimli-rs:master.

[fitzgen]

Very nice!