Make scalar multiplication type stable

[eschnett]

Also provide a unary + operator.

Closes #899.

[mateuszbaran]

Thanks for a patch, could you also add appropriate @inferred tests for this case?

[eschnett]

I see errors

Test Failed at /Users/eschnett/.julia/dev/StaticArrays/test/ambiguities.jl:19
  Expression: length(detect_ambiguities(Base, LinearAlgebra, StaticArrays)) <= allowable_ambiguities
   Evaluated: 61 <= 0
ERROR: LoadError: LoadError: There was an error during testing
in expression starting at /Users/eschnett/.julia/dev/StaticArrays/test/ambiguities.jl:13
in expression starting at /Users/eschnett/.julia/dev/StaticArrays/test/runtests.jl:38
ERROR: Package StaticArrays errored during testing

However, I already see them with the head of the master branch, it's not caused by my pull request.

[mateuszbaran]

These errors are a separate issue.

[mateuszbaran]

One last thing, could you bump version to 1.1.2?