Compile time sequences

[drug]

According to https://dlang.org/articles/ctarguments.html compile time 
sequences "...allow a programmer to operate on types, symbols and values..."
Phobos has `isType`/`isTypeTuple` traits, also `isExpressions` where 
expression may contain both values and symbols, but has no traits like 
`isSymbol` and `isValue`. I handle this by taking an address - values 
has no address, so I can distinct types, symbols and values, but it's 
not convenient/consistent way. I think it worths to add such traits, but 
probably I missed something?