Add more attributes

[r3v2d0g]

This PR adds four new struct attributes, and seven new field attributes. It also adds support for using #[prost(...)] multiple times on a field.

Struct attributes

• debug = bool (default: true): if false, prost won't implement Debug for the struct
• default = bool (default: true): if false, prost won't implement Default for the struct
• merge = bool (default: true): if false, prost will implement Message::merge_field and Message::merge to return an errror
• proto = "proto2" | "proto3" (default: "proto2"): changes the default behavior of message fields without required or optional specified to be optional when proto = "proto2" and required when proto = "proto3"

Field attributes

Scalar and message fields

T is the field's type. U is either the specified scalar or a type implementing Message.

• as_msg = "fn_or_closure": fn (field: &T) -> &U
• to_msg = "fn_or_closure": fn (field: &T) -> U
• from_msg = "fn_or_closure": fn (msg: U) -> T
• merge_msg = "fn_or_closure": fn (field: &mut T, msg: U)
• clear = bool (default: true): if false, doesn't clear this field when Message::clear is called

repeated fields

T is either the field's type (if as_msgs or to_msgs is set) or the T in Vec<T> (if as_msgs and to_msgs are not set). U is either the specified scalar or a type implementing Message.

• as_msg = "fn_or_closure": fn (elem: &T) -> &U
• to_msg = "fn_or_closure": fn (elem: &T) -> U
• as_msgs = "fn_or_closure": fn (field: &T) -> &Vec<U> or fn (field: &T) -> impl Iterator<Item = &U> + '_
• to_msgs = "fn_or_closure": fn (field: &T) -> Vec<U> or fn (field: &T) -> impl Iterator<U> + '_
• from_msg = "fn_or_closure": fn (msg: U) -> T
• merge_msg = "fn_or_closure": fn (elem_or_field: &mut T, msg: U)
• clear = bool (default: true): if false, doesn't clear this field when Message::clear is called

If the field is defined as packed or the scalar type is a number, then as_msgs and to_msgs should not return an Iterator.

If as_msgs or to_msgs is set, then as_msg and to_msg should not be set.

map = "key, value" fields

T is the value type of the map (the V in HashMap<K, V> and BTreeMap<K, V>). U is either the specified scalar (as value) or a type implementing Message.

• as_msg = "fn_or_closure": fn (field: &T) -> &U
• to_msg = "fn_or_closure": fn (field: &T) -> U
• from_msg = "fn_or_closure": fn (msg: U) -> T
• merge_msg = "fn_or_closure": fn (field: &mut T, msg: U)
• clear = bool (default: true): if false, doesn't clear this field when Message::clear is called

[LucioFranco]

This seems interesting, since its quite large its not extremely high priority on my list but I will get to this soon. Thanks for the patience.

[LucioFranco]

@r3v2d0g I am still interested in adding this to prost but would you be able to break these up into some smaller PRs. Thanks!