Also added comments for the members of SnapshotData and renamed
blob to v8_snapshot_blob_data for clarity.

Since V8 code cache encodes indices to the read-only space
it is safer to make sure that the code cache is generated in the
same heap used to generate the embdded snapshot. This patch
merges the code cache builder into the snapshot builder and
makes the code cache part of node::SnapshotData that is
deserialized into the native module loader during bootstrap.

…apshot