Unavailable I128 and U128 models

[oscbyspro]

This patch adds unavailable I128 and U128 models (#143).

The underlying Swift.Int128 and Swift.UInt128 models fail some tests. I have submitted some fixes that got merged into the Swift 6.1 branch. I'll try again later and make the I128 and U128 available when all tests pass successfully.

[oscbyspro]

Hm. I suppose it as to be 128-bit aligned for Element to equal Self.

Quoting the SE-0425 proposal:

The [U]Int128 types are 16B aligned on 64b targets¹ and have the same alignment as [U]Int64 on 32b targets.

64-bit alignment works for some purposes, but I'm not sure if it works for all purposes. Hm.

[oscbyspro]

Hm. I believe the actual requirement is something like this:

// where A: SystemsInteger & A.Element == A
// where B: SystemsInteger & B.Element == B

if A.size > B.size {
   MemoryLayout<A>.alignment >= MemoryLayout<B>.alignment
}  else if A.size < B.size {
   MemoryLayout<A>.alignment <= MemoryLayout<B>.alignment
}  else {
   MemoryLayout<A>.alignment == MemoryLayout<B>.alignment
}

Imagine U128 is 32-bit aligned, but U64 is 64-bit aligned:

// where  U64 is 64-bit aligned
// where U128 is 32-bit aligned

| 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 |
|--  64 --|--  64 --|--  64 --|--  64 --|
          |--  64 --|--  64 --|--  64 --|--  64 --|

| 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 |
|------- 128 -------|------- 128 -------|
     |------- 128 -------|------- 128 -------|
     |    |------- 128 -------|------- 128 -------|
     |
     └ U128 is NOT a [U64, U64]

It should be fine for U128 and U64 to both be 64-bit aligned, however:

// where  U64 is 64-bit aligned
// where U128 is 64-bit aligned

| 32 | 32 | 32 | 32 | 32 | 32 |
|--  64 --|--  64 --|
          |--  64 --|--  64 --|
          
| 32 | 32 | 32 | 32 | 32 | 32 |
|------- 128 -------|
          |------- 128 -------|
          |
          └ U128 is [U64, U64]

If same-size types have different alignment, then size is not enough info to reinterpret them:

// where U64_64 is 64-bit aligned
// where U64_32 is 32-bit aligned

| 32 | 32 | 32 | 32 |
|--  64 --|
          |--  64 --|
          
| 32 | 32 | 32 | 32 |
|--  64 --|
     |--  64 --|
     |    |--  64 --|
     |
     └ U64_32 is NOT a U64_64