multiprocess fuzzing

- New Features

-- Multiprocess Fuzzing

The fuzzer now is able to utilize multiple cores. This is controllable
with the `-j` build option. Limited fuzzing still uses one core.

-- Fuzzing Infinite Mode

When provided multiple tests, the fuzzer now switches between them and
prioritizes the most effective and interesting ones. Over time already
explored tests will become barely run compared to tests yielding new
inputs.

-- Crash Dumps

Crashing inputs are now saved to a file indicated by the crash message.
It is recommended to use these files to reproduce the crash using
`std.testing.FuzzInputOptions.corpus` and @embedFile.

- Design

Each fuzzing process is assigned an instance id which has the following
uses:
* In conjunction with the pc hash and running test index, they uniquely
  identify input files in the case of a crash.
* It is combined with the test seed for a unique rng seed.
* Instance 0 is solely responsible for syncing the filesystem corpus.

When new inputs are found, they are sent to the build server. It then
distributes the new input to the other instances. Each instance has a
concurrent poller managed by the test runner which sends received
inputs to libfuzzer. (note that this is affected by #31718 and so can
(rarely) deadlock)

For fuzzing infinite mode, the test runner now receives a list of tests
from the build server. The fuzzer runs tests in batches of one second,
approximated in cycles by the previous batch's run speed. Tests finding
new inputs or with few runs are given a higher run chance. The baseline
run chance is based off the recency of the last find and the number of
pcs the test has hit.

lib/std/Build.zig

@@ -128,6 +128,9 @@ pub const Graph = struct {
     random_seed: u32 = 0,
     dependency_cache: InitializedDepMap = .empty,
     allow_so_scripts: ?bool = null,
+    /// Steps should use `io` to limit the number of jobs, however in the case of
+    /// a single step spawning a fixed number of processes this can be used.
+    max_jobs: ?u32 = null,
     time_report: bool,
     /// Similar to the `Io.Terminal.Mode` returned by `Io.lockStderr`, but also
     /// respects the '--color' flag.