Optimize calculation of method extents

MethodMap.GetMethodExtents can be improved, the debug metadata schema has been updated to allow the optimization since the code was written.

See comment in code:

```
// We read and decode all sequence points in the file, which might be megabytes of data that need to be paged in.
// If we stored the primary document of single-document methods in a field of MethodBody table we would only need to decode
// sequence point of methods that span multiple documents to build a map from Document -> Methods.
// We can then defer decoding sequence points of methods contained in a specified document until requested.
```