precompile package callback (#341)

With this PR:
```
(ChainRulesCore) pkg> precompile
Precompiling project...
  1 dependency successfully precompiled in 2 seconds (1 already precompiled)

julia> @time using ChainRulesCore
  0.082255 seconds (157.13 k allocations: 9.737 MiB, 5.59% compilation time)
```

Before:
```
julia> @time using ChainRulesCore
  0.111551 seconds (443.47 k allocations: 25.618 MiB, 3.87% compilation time)
```

Removing the hook alltogether:
```
julia> @time using ChainRulesCore
  0.033357 seconds (36.37 k allocations: 2.399 MiB, 17.75% compilation time)
```

That's still more overhead than I'd like, so we should still discuss
whether we could avoid these hooks, but it does at least improve the current
situation somewhat.

Ref #340

Author: simeonschaub

src/precompile.jl

@@ -0,0 +1 @@
+precompile(_package_hook, (Base.PkgId,))

src/ruleset_loading.jl

@@ -1,7 +1,8 @@
 # Infastructure to support generating overloads from rules.
+_package_hook(::Base.PkgId) = refresh_rules()
 function __init__()
     # Need to refresh rules when a package is loaded
-    push!(Base.package_callbacks, pkgid -> refresh_rules())
+    push!(Base.package_callbacks, _package_hook)
 end
 
 # Holds all the hook functions that are invokes when a new rule is defined