We should at least log (a) the scripts we're processing in WarpOracle and (b) the resulting WarpSnapshot. This can probably just use the JIT spew mechanism for now, but later on we could add a structured spewer as well.

Adds code to log the scripts we're compiling. Also, with IONFLAGS=warp-snapshots
we now dump the contents of the WarpSnapshot at the end of the WarpOracle phase.
This will probably become more useful once we have CacheIR snapshots.

This also removes the CFG spew channel, it has been unused since bug 1595476.

Example warp-snapshots output:

WarpSnapshot (0x7ff27483a410)
------------------------------
globalLexicalEnv: 0x2e8060780040
globalLexicalEnvThis: 0xfffe2e8060780070

Script: self-hosted:200:18 (0x2e80607a26f0)
  moduleObject: 0x0
  isArrowFunction: 0
  environment: Function: callobject template 0x2e806078a280, named lambda template: 0x0

  WarpGetIntrinsic (offset 9, JSOp::GetIntrinsic)
    intrinsic: 0xfff9800000000000

  WarpGetIntrinsic (offset 65, JSOp::GetIntrinsic)
    intrinsic: 0xfffe2e80607a79c0

  WarpGetIntrinsic (offset 83, JSOp::GetIntrinsic)
    intrinsic: 0xfffe2e80607a7a00

  WarpLambda (offset 177, JSOp::Lambda)
    baseScript: 0x2e80607a2740
    flags: 0xa38
    nargs: 2

https://hg.mozilla.org/mozilla-central/rev/b94d7d99dad2