[Swift language features] Implement Swift structs in C#

[kotlarmilos]

Frozen structs

• Project basic frozen structs
• Project struct constructors
  • [Swift language features] Add support for methods marked with throws #2888
• Project struct instance methods
• Project struct static methods
• Project methods accepting struct as parameter
• Project non-init functions returning new instance of frozen struct

Non-frozen structs

• Project basic non-frozen structs
• [Swift language features] Add support for non-frozen structs #2870
• Project struct constructors
  • [Swift language features] Add support for methods marked with throws #2888
• Project struct instance methods
• Project struct static methods
• Project methods accepting struct as parameter
• [Swift language features] Project non-init functions returning new instance of non-frozen struct  #2885
  • Provide MarshalTo, MarshalFrom API [Swift language features] prototype for SwiftOptional #2896