Crash when reflecting a splice of a value

[katzdm]

The following program yields an ICE:

#include <experimental/meta>

constexpr auto s = ^[:std::meta::reflect_value(3):];

The bug was introduced during a recent refactor of reflect-expressions. This is probably a good opportunity to revisit how we parse and represent splices, and perhaps do a little refactoring.

[katzdm]

The whole ^[:expr:] syntax just adds wording and parsing complexity, while delivering basically zero value. Since it hasn't even been added to the paper, we're just going to refrain from doing so. I'll remove support for the syntax, rather than explicitly fix this bug.

[katzdm]

Removed support for this syntax in f26b7a.