Moved to a more linear layout which lends itself well to exposing an iterator.
Consumers of this iterator now just have to keep track of an index into a
homogenous sequence of bodies.

The new ZIR layout also enables giving switch prong items result locations
by storing the bodies of all items inside of the switch encoding itself.
There are some deliberate exceptions to this: enum literals and error values
are directly encoded as strings and number literals are resolved to comptime
values outside of the switch block. These special encodings exist to save
space and can easily be resolved during semantic analysis.

This commit also re-implements `AstGen` and `print_zir` for switch based on
the new layout and adds some additional information to the ZIR text repr.
Notably `switchExprErrUnion` has been merged into `switchExpr` to reduce
code duplication.

The rules around allowing an unreachable `else` prong in error switches are
also refined by this commit, and enforced properly based on the actual AST.
The special cases are listed exhaustively below:

`else => unreachable,`
`else => return,`
`else => |e| return e,` (where `e` is any identifier)

Additionally `{...} => comptime unreachable,` prongs are marked to support
future features (refer to next couple of commits).

Also fixes 'value with comptime-only type depends on runtime control flow'
error for labeled error switch statements by surrounding the entire expr
with a common block to break to (see previous commits for details).