message: Update default codegen-units
  It's been 16 since rustc 1.24.
diff: @@ -299,7 +299,7 @@ lto = false # Link Time Optimization usually reduces size of binaries
  # be passed
  debug-assertions = true # controls whether debug assertions are enabled
  # (e.g. debug_assert!() and arithmetic overflow checks)
  -codegen-units = 1 # if > 1 enables parallel code generation which improves
  +codegen-units = 16 # if > 1 enables parallel code generation which improves
  # compile times, but prevents some optimizations.
  # Passes `-C codegen-units`.
  panic = 'unwind' # panic strategy (`-C panic=...`), can also be 'abort'
  @@ -314,7 +314,7 @@ debug = false
  rpath = false
  lto = false
  debug-assertions = false
  -codegen-units = 1
  +codegen-units = 16
  panic = 'unwind'
  incremental = false
  overflow-checks = false
  @@ -326,7 +326,7 @@ debug = 2
  rpath = false
  lto = false
  debug-assertions = true
  -codegen-units = 1
  +codegen-units = 16
  panic = 'unwind'
  incremental = true
  overflow-checks = true
  @@ -338,7 +338,7 @@ debug = false
  rpath = false
  lto = false
  debug-assertions = false
  -codegen-units = 1
  +codegen-units = 16
  panic = 'unwind'
  incremental = false
  overflow-checks = false
  @@ -350,7 +350,7 @@ debug = 2
  rpath = false
  lto = false
  debug-assertions = true
  -codegen-units = 1
  +codegen-units = 16
  panic = 'unwind'
  incremental = true
  overflow-checks = true
  