ArcIntern: add from_str() specialization for ArcIntern<String>

[eliad-wiz]

This is useful in order to allow looking for interned str without having to create owned value first.

simple benchmarks show nice improvement in case the str was already interned:

    cached/String/short     time:   [13.598 ms 13.783 ms 13.994 ms]
                            change: [-0.5323% +1.5518% +3.7562%] (p = 0.15 > 0.05)
                            No change in performance detected.
    Found 14 outliers among 100 measurements (14.00%)
      7 (7.00%) high mild
      7 (7.00%) high severe
    cached/String/short-from_ref
                            time:   [13.582 ms 13.757 ms 13.955 ms]
                            change: [-1.5349% +0.4010% +2.5499%] (p = 0.70 > 0.05)
                            No change in performance detected.
    Found 12 outliers among 100 measurements (12.00%)
      3 (3.00%) high mild
      9 (9.00%) high severe
    cached/String/short-from_str
                            time:   [9.3047 ms 9.4529 ms 9.6175 ms]
                            change: [-1.9867% +0.4488% +2.9423%] (p = 0.72 > 0.05)
                            No change in performance detected.
    Found 15 outliers among 100 measurements (15.00%)

if the string is not interned we do end up with additional lookup, but other functions (new/from_ref) can be used if that's an issue.