Add URL macro to create valid non-optional URLs.

[NikolaiRuhe]

Add a #URL macro that provides compile time checked URL literals.

Motivation:

This is similar in spirit to compile time checked regular expressions. For a literal Regex the compiler makes sure that the regex has no syntactic errors by parsing it at compile time. In case of a syntax error the compiler emits an error (and does not produce any code).

When the regex parses without error a non-optional, non-throwing initializer is inserted. During runtime the Regex has to be parsed again to create the actual instance but it is known to not throw an error.

This behavior, where the compiler checks the validity of an initializer's arguments would be helpful for many other types, for example a SemVer type that checks for a valid semantic version.

In these cases macros seem like a good workaround (until we get statically checked arguments).

[DougGregor]

Very cool, thank you!