perf: Avoid allocating some redundant Vec's during encoding

[Marwes]

A couple of small and simple performance improvements I could do easily while looking through the code. There is definitely more that could be done here towards avoiding allocations but I didn't want to get sidetracked any further and that will be easier to build on top of the &mut Vec style as well.

 name                                        before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 bench_encode_pipeline                       498,750         515,194              16,444    3.30%   x 0.97
 bench_encode_pipeline_nested                329,036         225,092            -103,944  -31.59%   x 1.46
 bench_long_pipeline                         1,819,621       1,697,848          -121,773   -6.69%   x 1.07
 bench_simple_getsetdel                      89,985          90,718                  733    0.81%   x 0.99
 bench_simple_getsetdel_pipeline             35,303          35,811                  508    1.44%   x 0.99
 bench_simple_getsetdel_pipeline_precreated  34,209          33,526                 -683   -2.00%   x 1.02

[mitsuhiko]

lgtm!