allow configuration of error set tag type via a public const in the root source file

[andrewrk]

Right now error sets are hardcoded to codegen to a u16.

Ideally, zig would choose an appropriate sized integer based on the total number of error values in the compilation, but the user may do @sizeOf(SomeStruct) which has an error set type as a field before the entire compilation finishes analysis. This would create a circular dependency.

One idea is to have the error set integer type have a default of u32, but in the same way that you can override the default panic handler in the root source file, you can override the error set integer type in the root source file by doing:

pub const ErrorIntegerType = u8;

Then you'd get a compile error if zig ran out of bits when choosing error values.

[daurnimator]

Further proposal:

For performance reasons it would be nice if errno values mapped to their same number in the error set, e.g. EPERM -> 1, that way most errno processing code doesn't need a lookup table; this saves in both runtime cost and binary size (smaller error lookup tables).

One way we could do this is by allowing the ErrorSet type to be declared via an enum in the root file (rather than the original proposal of an integer type). Lets make the restriction that it must be a non-exhaustive enum.

A straightforward setup, just declaring the error set size:

pub const ErrorSetType = enum(u8) { _ };

Mapping errno:

pub const ErrorSetType = enum(u32) {
  PermissionDenied = std.os.EPERM,
  FileNotFound = std.os.ENOENT,
  ProcessNotFound = std.os.ESRCH,
  // etc.
  _,
};