support proper 32-byte RNG seeds

• add optional $.help to argument err of error throw
• parse prng_seed: [u8; 32] in rng and inbreeding
• compute the "local" seed by hashing the "global" seed, the strategy and the iteration index
• pass --prng-seed: string, a 64-char long seed to inbreeding run

.nushell/error.nu

-export def "error throw" [err: record<err: string, label: string, span: record<start: int, end: int>>] {
+export def "error throw" [err: record<
+    err: string,
+    label: string,
+    span: record<start: int, end: int>,
+    # help: string?,
+>] {
     error make {
         msg: $"(ansi red_bold)($err.err)(ansi reset)",
         label: {
             text: $err.label,
             span: $err.span,
         },
+        help: $err.help?,
     }
 }