Currently, we don't parse string templates in normal strings.

As a first step, we could allow at least arguments which are already strings like:

val domain = "https://api.effekt-lang.org"
val user = "effekt"
val resourceId = 42

"GET ${domain}/users/${user}/resource/${resourceId.show}"

This would involve:

1. changing the parser (allowing ${ ... } to appear in normal strings)
2. allowing string literals to be Template[...] in source.Tree
3. piping them through Namer
4. in Typer, making sure all of the splices are Strings
5. and then when transforming into Core, just concatenate them for now into a Core string literal

There are some interesting detours/shortcuts:

• try using show on the splices instead (probably horrible to use because of the error messages)
• desugar to string concatenation directly in the parser
• have the templates as a primitive instead of string concatenation; just define def infixConcat(s1, s2) = "${s1}${s2}"