Interpolation struct names and location

[Steven Schveighoffer]

[DIP 1036e](https://github.com/dlang/dmd/pull/15715) has been 
merged and is going to be in the next release (set to be cut on 
March 1).

Once this is upon the world, it will be very difficult to change 
the names of the types used for these things. I want to make sure 
everyone has a chance to think about this and suggest what they 
think.

The way the lowering is set up, you do NOT have to import 
`core.interpolation` to *use* them. Most of the time, you will be 
only doing that in order accept the types as function parametrers.

However, [these 
names](https://github.com/dlang/dmd/blob/98443676dcfa32f7234ebab6ca9f925e1d3cd736/druntime/src/core/interpolation.d#L102) are rather verbose. And every function that accepts these sequences is going to be mangled with some pretty long names.

I propose we discuss both for where these types should live and 
what they should be called. I personally think we might want to 
move these things to object.d (yes, I know object.d is big, but 
these are not big or complex types), and rename them something 
more succinct.

Something like `iHeader`, `iFooter`, `iExpr`, `iLiteral` would be 
descriptive enough (everyone is going to be calling these 
istrings, even though they are Interpolation Expression 
Sequences).

This is the time to debate, before it's released!

-Steve