Remove MaybeDone from impl Join for array

[yoshuawuyts]

Ref #22. Moves us away from MaybeDone for array::join.

Performance

Performance appears to remain about equal as it was before, with a performance diff which can easily be attributed to noise:

❯ critcmp main patch  -f array::join
group               main                                   patch
-----               ----                                   -----
array::join 10      1.03   589.1±26.71ns        ? ?/sec    1.00   571.7±12.23ns        ? ?/sec
array::join 100     1.00      3.7±0.03µs        ? ?/sec    1.05      3.8±0.16µs        ? ?/sec
array::join 1000    1.00     33.0±0.34µs        ? ?/sec    1.03     33.8±1.01µs        ? ?/sec