Adds non allocating alternatives to ZON parse functions

[MasonRemaley]

Adds the following functions to std.zon.parse:

• fromSliceFlat
• fromZoirFlat
• fromZoirNodeFlat

These are identical to the existing functions with "flat" stripped from the name, but they assert at compile time that the result type doesn't contain a pointer and therefore doesn't need to be freed. As such, the latter two functions do not require an allocator as an argument. (fromSlice still does as it needs to allocate the Ast and Zoir.)

I'm open to alternative suggestions for the names. I think "flat" as in "flat in memory" is pretty reasonable, though it could be misinterpreted as to do with nesting. I would have simply named them noAlloc, but this is confusing since fromSliceFlat does in fact create (temporary) allocations.

Replaces #22835, CC @NicoElbers (thanks for the reminder that I wanted to do this!)