1. Reduce allocations when reading from FileStream.
2. Pre-compute checksum if possible.
3. Reduce allocations for lazily computed checksums.
4. Avoid LOH allocations for large source text.
5. Update EditorTextFactory to optimize for reads from temporary storage

Perf results for the Picasso rebuild scenario:
184 MB total allocations saved. Including: 78 MB of char[], 73 MB of Byte[], 9 MB of string. Also, 35 MB of LOH allocations were eliminated.
 (changeset 1412085)