Introduce a pattern that would allow types to participate in fixed statements.

• Proposal added (https://github.com/dotnet/csharplang/blob/master/proposals/fixed-sized-buffers.md)
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

LDM history:

• briefly discussed in LDM on 2/7/2018 with some interest, identifying champion.
• https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-04-03.md#fixed-size-buffers
• https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-04-10.md
• https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-01.md#fixed-size-buffers
• https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-03.md#inline-arrays
• https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-17.md#inline-arrays

Tagging @VSadov @jaredpar