Rollup merge of #67253 - elichai:2019-12-fmt, r=Dylan-DPC

Rollup merge of #67253 - elichai:2019-12-fmt, r=Dylan-DPC

Add more delegations to the fmt docs and add doctests

HI,
this is a continuation to #67021
I replaced the `Debug` example with one that use the `Debug*` helpers so that padding etc will work too.
I also added asserts for the doctests as @RalfJung asked :)

The only thing I left with the `write!` macro is the `Display` example as I didn't know if there's a better way to do that.

r? @QuietMisdreavus