fix: use var_{id} as local variable name to avoid the name conflict with tuple struct

[Ggiggle]

Motivation

Since we will generate the enum type in thrift as newtype in rust, that is a tuple struct, there comes a compile error that let bindings cannot shadow tuple structs when the field name in idl is same as the tuple struct.
For example, the idl below

enum Index {
    A = 0x01,
    B = 0x10,
}

struct Request {
    1: required Index Index,
    2: Index index,
}

service Test {
   void test_enum_var_type_name_conflict (1: Request req);
}

Solution

We will generate a local variable name with the new rule var_{field.id} rather than the rust name of the field to avoid the conflict.