Avoid moving result of read_attribute

[philipc]

 name                                        before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 context_new_and_query_location_rc           2,490,696       2,447,360            -43,336   -1.74%   x 1.02
 context_new_and_query_location_slice        772,800         773,439                  639    0.08%   x 1.00
 context_new_and_query_with_functions_rc     3,068,885       3,021,044            -47,841   -1.56%   x 1.02
 context_new_and_query_with_functions_slice  1,180,680       1,144,850            -35,830   -3.03%   x 1.03
 context_new_parse_functions_rc              37,416,109      30,829,671        -6,586,438  -17.60%   x 1.21
 context_new_parse_functions_slice           26,969,411      24,411,324        -2,558,087   -9.49%   x 1.10
 context_new_parse_lines_rc                  11,583,804      11,516,118           -67,686   -0.58%   x 1.01
 context_new_parse_lines_slice               7,296,223       7,345,699             49,476    0.68%   x 0.99
 context_new_rc                              2,311,116       2,247,925            -63,191   -2.73%   x 1.03
 context_new_slice                           626,076         621,520               -4,556   -0.73%   x 1.01
 context_query_location_rc                   726,244         727,900                1,656    0.23%   x 1.00
 context_query_location_slice                722,684         723,472                  788    0.11%   x 1.00
 context_query_with_functions_rc             4,346,361       4,280,144            -66,217   -1.52%   x 1.02
 context_query_with_functions_slice          4,328,587       4,331,231              2,644    0.06%   x 1.00
 new                                         29,658          28,688                  -970   -3.27%   x 1.03
 new_unresolved_and_resolve_separate         29,812          29,250                  -562   -1.89%   x 1.02
 trace_and_resolve_callback                  14,333          13,607                  -726   -5.07%   x 1.05
 trace_and_resolve_separate                  12,279          11,270                -1,009   -8.22%   x 1.09

This is the equivalent of gimli-rs/gimli#456.

[coveralls]

Coverage decreased (-0.6%) to 70.529% when pulling 292d195 on philipc:entries-raw-ref into af29dba on gimli-rs:master.

[fitzgen]

👍