Add option to add field setters to PackedTypes

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Move Table to AdvancedTable(comptime mutable: bool) type with flatbuffer: if (mutable) []u8 else []const u8. Make pub const Table = AdvancedTable(false); and pub const MutableTable = AdvancedTable(true); (with mixed in setter methods).

Then codegen setters for the following types:

• Scalars
• Strings (with len checking against previous string?)
• Vector + array elements
• Is there a way to do unions?