add new language tour section on type inference

[SethTisue]

original draft by @travissarles. touched up by me

replaces #737

[SethTisue]

oh, I'll have to do something about this:

[tut] compiling: ./_tour/type-inference.md
[tut] *** Error reported at /drone/src/github.com/scala/docs.scala-lang/_tour/type-inference.md:64: failure was asserted but no failure occurred
obj: Null = null

(tut:fail expects everything inside to fail. so if you want to show some failing code that requires some correct code beforehand, you have to use two blocks)

[SethTisue]

I'm going to go ahead and merge, since #737 was open for feedback for many months.