support structs

[andrewrk]

struct Aoeu {
    field: i32,
    field2: bool,
}

• struct on the stack, read / write fields
• accept a struct pointer as a parameter. read / write fields
• debug information for structs
• struct which has a member which is a pointer to same struct
• struct which has a member which is another struct declared later
• circular reference struct is an error
• invalid field access error test
• assign to invalid field error test
• assign one byvalue struct to another
• struct initializer syntax
• assign field to constant struct error
• redefinition of struct error
• pub vs private vs export structs
• error for modifying a byval parameter
• error for byval struct parameter in exported function (since we don't yet support it)
• error for missing field in struct value expression
• error for incorrect field in struct value expression
• error for taking mutable address of const struct field