Python: Move min/maxParameter methods to Function

These seem generally useful outside of points-to, and so it might be
better to add them to the `Function` class instead.

I took the liberty of renaming these to say `Arguments` rather than
`Parameters`, as this is more in line with the nomenclature that we're
using elsewhere. (The internal points-to methods retain the old names.)

I'm somewhat ambivalent about the behaviour of `getMaxParameters` on
functions with `*varargs`. The hard-coded `INT_MAX` return value is
somewhat awkward, but the alternative (to only have the predicate
defined when a specific maximum exists) seems like it would potentially
cause a lot of headaches.

Author: tausbn

python/ql/lib/semmle/python/Function.qll

@@ -163,6 +163,18 @@ class Function extends Function_, Scope, AstNode {
       ret.getValue() = result.getNode()
     )
   }
+
+  /** Gets the minimum number of positional arguments that can be correctly passed to this function. */
+  int getMinArguments() {
+    result = count(this.getAnArg()) - count(this.getDefinition().getArgs().getADefault())
+  }
+
+  /** Gets the maximum number of positional arguments that can be correctly passed to this function. */
+  int getMaxArguments() {
+    if exists(this.getVararg())
+    then result = 2147483647 // INT_MAX
+    else result = count(this.getAnArg())
+  }
 }
 
 /** A def statement. Note that FunctionDef extends Assign as a function definition binds the newly created function */

python/ql/lib/semmle/python/objects/ObjectAPI.qll

@@ -738,21 +738,9 @@ class PythonFunctionValue extends FunctionValue {
     else result = "function " + this.getQualifiedName()
   }
 
-  override int minParameters() {
-    exists(Function f |
-      f = this.getScope() and
-      result = count(f.getAnArg()) - count(f.getDefinition().getArgs().getADefault())
-    )
-  }
+  override int minParameters() { result = this.getScope().getMinArguments() }
 
-  override int maxParameters() {
-    exists(Function f |
-      f = this.getScope() and
-      if exists(f.getVararg())
-      then result = 2147483647 // INT_MAX
-      else result = count(f.getAnArg())
-    )
-  }
+  override int maxParameters() { result = this.getScope().getMaxArguments() }
 
   /** Gets a control flow node corresponding to a return statement in this function */
   ControlFlowNode getAReturnedNode() { result = this.getScope().getAReturnValueFlowNode() }