Support "partially derive"

For example such case:

REG1:
   description: REG1
   addressOffset: 0x0
   resetValue: 0x0
   fields:
        FIELD1: {}
        FIELD2: {}
        FIELD3: {}
REG2:
   addressOffset: 0x4
   derivedFrom: REG1
REG3:
   description: REG3
   addressOffset: 0x8
   resetValue: 0xF0
   derivedFrom: REG1

we should get something like this:

/// REG1
pub mod reg1 {
  // Fields
}
pub struct Reg1Spec;
/// REG1
pub type Reg1 = Reg<Reg1Spec>;

/// REG1
pub use reg1 as reg2; // same module
/// REG1
pub use Reg1 as Reg2; // same register

/// REG3
pub use reg1 as reg3;
pub struct Reg3Spec; // same module
/// REG3
pub type Reg3 = Reg<Reg3Spec>; // different registers

cc @Emilgardis

No file or test is named; begin by finding the Rust generator path that handles derivedFrom. Use the REG1/REG2/REG3 YAML example to compare current output with the requested module and type aliases, and add coverage showing that partial derivation is preserved.