Provide APIs to get a pointer/value out of a heap given a heap index

[jkoritzinsky]

Today, DNMD does not provide an operation to get a value from a heap at a given index. DNMD only provides an API to read a heap value from a column that points into a heap. This works for most scenarios, but the direct "read from the heap at this index" is needed for a few scenarios:

• The Mono embedding API surface exposes this operation (get a pointer to the offset into a heap)
• The Document table's Name column's format specifies offsets into the blob heap for blobs to be read to form the path to the document.
• The ImportScope table's Imports column's format specifies offsets into the blob heap for the alias and target-namespace data in the blob.

As the DNMD API exists today, these operations cannot be implemented.

[jkoritzinsky]

We discussed offline and came to the following conclusion:

• The Mono embedding API surface concern is not interesting unless we must keep the existing API surface (at which point we have a few options, such as including a private header just for Mono that we don't ship if we ever integrate into the runtimes)
• We can provide specialized functions/structs for the known annoying blobs (Document.Name, ImportScope.Imports) to handle them internally without exposing this API.

[jkoritzinsky]

We will face a similar problem for all Portable PDB blobs that use compressed/uncompressed integers (as we don't expose the API to parse them).