rpc: separate out request and response structures

Mostly as is, no real effort done yet to optimize them, so there are still a
lot of duplicates there, but at least we sort them out into different smaller
packages.

pkg/rpc/request/params.go

@@ -0,0 +1,26 @@
+package request
+
+type (
+	// Params represents the JSON-RPC params.
+	Params []Param
+)
+
+// Value returns the param struct for the given
+// index if it exists.
+func (p Params) Value(index int) (*Param, bool) {
+	if len(p) > index {
+		return &p[index], true
+	}
+
+	return nil, false
+}
+
+// ValueWithType returns the param struct at the given index if it
+// exists and matches the given type.
+func (p Params) ValueWithType(index int, valType paramType) (*Param, bool) {
+	if val, ok := p.Value(index); ok && val.Type == valType {
+		return val, true
+	}
+
+	return nil, false
+}