FIR2IR: read & set dispatch & extension receivers

NB: this commit includes receiver-based assertions yet failing for some tests

compiler/testData/ir/irText/singletons/enumEntry.fir.txt

@@ -25,6 +25,7 @@ FILE fqName:<root> fileName:/enumEntry.kt
           $this: VALUE_PARAMETER name:<this> type:<root>.Z.ENTRY.A
           BLOCK_BODY
             CALL 'public final fun test (): kotlin.Unit declared in <root>.Z.ENTRY' type=kotlin.Unit origin=null
+              $this: GET_VAR '<this>: <root>.Z.ENTRY declared in <root>.Z.ENTRY' type=<root>.Z.ENTRY origin=null
         FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:kotlin.Boolean
           overridden:
             public open fun equals (other: kotlin.Any?): kotlin.Boolean declared in kotlin.Any