Enable precompilation (#2456)

Run some `precompile` statements generated using SnoopCompile in the precompilation
phase.

In addition to this, the new `DataFrames.precompile(true)` function
precompiles all methods that are used when running tests.
This is useful to run benchmarks like H2Oai where the time of the first run should
include the time needed to specialize on a particular operation on a particular
dataset, but not the time needed for the more general compilation of common parts.
It could also be useful for users who would like to generate a precompiled image.

NEWS.md

@@ -114,4 +114,8 @@
 * limit the maximal display width the output can use in `text/plain` before
   being truncated (in the `textwidth` sense, excluding `…`) to `32` per column
   by default and fix a corner case when no columns are printed in situations when
-  they are too wide ([2403](https://github.com/JuliaData/DataFrames.jl/pull/2403))
+  they are too wide ([#2403](https://github.com/JuliaData/DataFrames.jl/pull/2403))
+* Common methods are now precompiled to improve responsiveness the first time a method
+  is called in a Julia session. Precompilation takes up to 30 seconds
+  after installing the package
+  ([#2456](https://github.com/JuliaData/DataFrames.jl/pull/2456)).

src/DataFrames.jl

@@ -126,4 +126,7 @@ include("deprecated.jl")
 
 include("other/tables.jl")
 
+include("other/precompile.jl")
+precompile()
+
 end # module DataFrames