Cow-ify ActionMeta

[FelixMcFelix]

Mooves ActionMeta from a String->String map to something capable of holding &'static strs as both keys and values. This only really affects slowpath traffic, but this leads to fewer allocations when traversing that path.

Split out of #805, where this wasn't a necessary part of the fix.

[FelixMcFelix]

Roughly demonstrating the impact via cargo ubench:

kyle@KyleOxide opte % cargo ubench V4-Tcp-OUT-1400B
   Compiling opte v0.1.0 (/Users/kyle/gits/opte/lib/opte)
   Compiling oxide-vpc v0.1.0 (/Users/kyle/gits/opte/lib/oxide-vpc)
   Compiling opte-test-utils v0.1.0 (/Users/kyle/gits/opte/lib/opte-test-utils)
   Compiling opte-bench v0.1.0 (/Users/kyle/gits/opte/bench)
    Finished `release-lto` profile [optimized + debuginfo] target(s) in 44.04s
     Running benches/userland.rs (target/release-lto/deps/userland-49b68a123c6927a2)
parse/ULP-FastPath/wallclock/V4-Tcp-OUT-1400B
                        time:   [44.552 ns 45.245 ns 45.835 ns]
                        change: [−0.9153% +1.0190% +2.8656%] (p = 0.30 > 0.05)
                        No change in performance detected.
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high mild

process/ULP-FastPath/wallclock/V4-Tcp-OUT-1400B
                        time:   [164.34 ns 165.54 ns 167.63 ns]
                        change: [−2.5020% −1.3224% −0.2918%] (p = 0.01 < 0.05)
                        Change within noise threshold.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) high mild
  4 (4.00%) high severe

parse/ULP-SlowPath/wallclock/V4-Tcp-OUT-1400B
                        time:   [44.168 ns 44.898 ns 45.498 ns]
                        change: [−2.1795% −0.2915% +1.6479%] (p = 0.77 > 0.05)
                        No change in performance detected.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

process/ULP-SlowPath/wallclock/V4-Tcp-OUT-1400B
                        time:   [192.16 ns 193.71 ns 195.68 ns]
                        change: [−32.299% −15.558% +4.0681%] (p = 0.16 > 0.05)
                        No change in performance detected.
Found 14 outliers among 100 measurements (14.00%)
  5 (5.00%) high mild
  9 (9.00%) high severe

process/ULP-FastPath/alloc_ct/V4-Tcp-OUT-1400B
                        time:   [3.0000  3.0000  3.0000 ]
                        change: [+0.0000% +0.0000% +0.0000%] (p = NaN > 0.05)
                        No change in performance detected.

process/ULP-SlowPath/alloc_ct/V4-Tcp-OUT-1400B
                        time:   [35.000  35.000  35.000 ]
                        change: [−18.605% −18.605% −18.605%] (p = 0.00 < 0.05)
                        Performance has improved.

process/ULP-FastPath/alloc_sz/V4-Tcp-OUT-1400B
                        time:   [264.00 B 264.00 B 264.00 B]
                        change: [+0.0000% +0.0000% +0.0000%] (p = NaN > 0.05)
                        No change in performance detected.

process/ULP-SlowPath/alloc_sz/V4-Tcp-OUT-1400B
                        time:   [6635.0 B 6635.0 B 6635.0 B]
                        change: [−1.1177% −1.1177% −1.1177%] (p = 0.00 < 0.05)
                        Performance has improved.

The baseline slowpath counts are 43 allocs and 6710B.