Make reused local initialization optional

The local reuse changes in 4.3.1 are reasonable and safe.  However, as a library for crafting IL this safety feature should be optional; the author should be able to take responsibility for initializing the local before reading.

Examples:

---
Safe default:
```csharp
using (var a = e1.DeclareLocal(typeof(int), "a"))
{
    // do something with local variable a
}

using (var b = e1.DeclareLocal(typeof(int), "b"))
{
    e1.LoadConstant(1);
    e1.StoreLocal(b);
    // do something with local variable b
}
```
IL:
```
ldc.i4.0
stloc.0
ldc.i4.1
stloc.0
```

---
Opt out:
```csharp
using (var a = e1.DeclareLocal(typeof(int), "a"))
{
    // do something with local variable a
}

using (var b = e1.DeclareLocal(typeof(int), "b", initializeReused: false))
{
    e1.LoadConstant(1);
    e1.StoreLocal(b);
    // do something with local variable b
}
```
IL:
```
ldc.i4.1
stloc.0
```