Refactoring: Extract operation parameters

With the addition of block parameters to operations with #361, it might be worthwhile to extract the operation parameters into its own class for better readability:

```scala
case BlockLiteral(tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam], body: Stmt)
...
case class Operation(id: IdDef, tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam], ret: Effectful) extends Definition
...
case class OpClause(id: IdRef, tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam], ret: Option[Effectful], body: Stmt, resume: IdDef) extends Reference
...
case FunDef(id: IdDef, tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam], ret: Option[Effectful], body: Stmt)
...
case ExternDef(capture: CaptureSet, id: IdDef, tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam], ret: Effectful, body: Template[Term]) extends Def
```

to

```scala
case BlockLiteral(params: Params, body: Stmt)
...
case class Operation(id: IdDef, params: Params, ret: Effectful) extends Definition
...
case class OpClause(id: IdRef, params: Params, ret: Option[Effectful], body: Stmt, resume: IdDef) extends Reference
...
case FunDef(id: IdDef, params: Params, ret: Option[Effectful], body: Stmt)
...
case ExternDef(capture: CaptureSet, id: IdDef, params: Params, ret: Effectful, body: Template[Term]) extends Def
...
case class Params(tparams: List[Id], vparams: List[ValueParam], bparams: List[BlockParam])
```