Added sym"..." string macro for making Symbols

[andyferris]

A new non-standard string literal using sym_str macro to construct Symbols. Supports interpolation.

This is the sibling PR of #32408, and is the product of the South-West Brisbane (Australia) edition of JuliaCon Hackathon 2019.

Some examples:

julia> sym"a"
:a

julia> sym"a#"
sym"a#"

julia> i = 10
10

julia> sym"a_$i"
:a_10

Co-authored-by: Chris Foster chris42f@gmail.com

[andyferris]

c.f. #32408 (comment)

[JeffBezanson]

This can already be done with Symbol("a"), so it's not super necessary (though we need to be able to constant fold that). We more urgently need a syntax for non-quoted non-standard symbols.

[KristofferC]

We more urgently need a syntax for non-quoted non-standard symbols.

We have that with var now. It seems the Symbol("") constructor is perhaps enough for the other use case.