inline arithmetic between chars and Integers

[KristofferC]

Arithmetic between Chars and Integers is often used in quite tight parts of parsing and other string / char manipulations.

As an example

Before

julia> @btime uppercase($(Ref('a'))[])
  5.332 ns (0 allocations: 0 bytes)
'A': ASCII/Unicode U+0041 (category Lu: Letter, uppercase)

After

julia> @btime uppercase($(Ref('a'))[])  
  3.858 ns (0 allocations: 0 bytes)
'A': ASCII/Unicode U+0041 (category Lu: Letter, uppercase)