Better String support at compile-time

[Iltotore]

When evaluating a String at compile-time, concatenated Strings are supported:

"abc" + "def"

but these alternatives are not supported:

Automatic toString

"abc" + 123

String interpolation

inline val x = 123
"abc$x"

When #227 is completed, it would improve usability of String constraints at compile-time.

[Iltotore]

Unfortunately, Dotty does very less desugaring/reduction for these two examples. Supporting these alternatives would imply adding much complexity for little gains.

It seems that this case is already handled by scala.quoted.FromExpr which what the new ExprDecoder is based on so adding interpolation support, while less trivial than concatenation, might still be possible.