FIP-0076 Add Market::GetDealSector method (#852)

FIPS/fip-0076.md

@@ -542,6 +542,25 @@ struct OnMinerSectorsTerminateParams {
 The sector is removed from the `ProviderSectors` mapping, if present.
 Any deals mapped to that sector are marked as terminated, and subsequent processing deferred to cron.
 
+#### GetDealSector
+
+A new FRC-0042 exported method "GetDealSector" (method 2611213344) returns the sector number in which
+a deal's data is stored, while a deal is active.
+If a deal is published but not yet activated, aborts with `EX_DEAL_NOT_ACTIVATED = 33`.
+If a deal is not found, aborts with the same exit codes as "GetDealActivation".
+
+```
+// "Transparent" CBOR-encoding, the singleton field is encoeded directly.
+struct GetDealSectorParams {
+    DealID: DealID,
+}
+
+// "Transparent" CBOR-encoding, the singleton field is encoeded directly.
+struct GetDealSectorReturn {
+    SectorNumber: SectorNumber,
+}
+```
+
 ### Migration
 
 The built-in market actor's `ProviderSectors` mapping is initialised from the existing deal state