Consider adding finer-grain control over field layout

[densh]

Currently we only support automatically aligned C-like struct layout. For some use-cases it might beneficial to provide more fine-grain control then that:

• Let user specify alignment of specific fields
• Let user provide specific offsets for given fields
• Let user define unaligned off-heap classes

From UI standpoint it might look like a few extra annotations (e.g. @unaligned, @align(n), @offset(n))