wgengine/filter: use IPSet for localNets instead of prefixes.

Part of #1177, preparing for doing fancier set operations on
the allowed local nets.

Signed-off-by: David Anderson <danderson@tailscale.com>

go.sum

@@ -317,6 +317,8 @@ go.uber.org/atomic v1.7.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
 go.uber.org/multierr v1.6.0/go.mod h1:cdWPpRnG4AhwMwsgIHip0KRBQjJy5kYEpYjJxpXp9iU=
 go4.org/intern v0.0.0-20210101010959-7cab76ca296a h1:28p852HIWWaOS019DYK/A3yTmpm1HJaUce63pvll4C8=
 go4.org/intern v0.0.0-20210101010959-7cab76ca296a/go.mod h1:vLqJ+12kCw61iCWsPto0EOHhBS+o4rO5VIucbc9g2Cc=
+go4.org/intern v0.0.0-20210108033219-3eb7198706b2 h1:VFTf+jjIgsldaz/Mr00VaCSswHJrI2hIjQygE/W4IMg=
+go4.org/intern v0.0.0-20210108033219-3eb7198706b2/go.mod h1:vLqJ+12kCw61iCWsPto0EOHhBS+o4rO5VIucbc9g2Cc=
 go4.org/mem v0.0.0-20201119185036-c04c5a6ff174 h1:vSug/WNOi2+4jrKdivxayTN/zd8EA1UrStjpWvvo1jk=
 go4.org/mem v0.0.0-20201119185036-c04c5a6ff174/go.mod h1:reUoABIJ9ikfM5sgtSF3Wushcza7+WeD01VB9Lirh3g=
 go4.org/unsafe/assume-no-moving-gc v0.0.0-20201222175341-b30ae309168e/go.mod h1:FftLjUGFEDu5k8lt0ddY+HcrH/qU/0qk+H8j9/nTl3E=
@@ -559,6 +561,8 @@ honnef.co/go/tools v0.1.0 h1:AWNL1W1i7f0wNZ8VwOKNJ0sliKvOF/adn0EHenfUh+c=
 honnef.co/go/tools v0.1.0/go.mod h1:XtegFAyX/PfluP4921rXU5IkjkqBCDnUq4W8VCIoKvM=
 inet.af/netaddr v0.0.0-20210105212526-648fbc18a69d h1:6f0242aW/6x2enQBOSKgDS8KQNw6Tp7IVR8eG3x0Jc8=
 inet.af/netaddr v0.0.0-20210105212526-648fbc18a69d/go.mod h1:jPZo7Jy4nke2cCgISa4fKJKa5T7+EO8k5fWwWghzneg=
+inet.af/netaddr v0.0.0-20210222205655-a1ec2b7b8c44 h1:p7fX77zWzZMuNdJUhniBsmN1OvFOrW9SOtvgnzqUZX4=
+inet.af/netaddr v0.0.0-20210222205655-a1ec2b7b8c44/go.mod h1:I2i9ONCXRZDnG1+7O8fSuYzjcPxHQXrIfzD/IkR87x4=
 inet.af/peercred v0.0.0-20210216231719-993aa01eacaa h1:6qseJO2iNDHl+MLL2BkO5oURJR4A9pLmRz11Yf7KdGM=
 inet.af/peercred v0.0.0-20210216231719-993aa01eacaa/go.mod h1:VZeNdG7cRIUqKl9DWoFX86AHyfYwdb4RextAw1CAEO4=
 k8s.io/api v0.16.13/go.mod h1:QWu8UWSTiuQZMMeYjwLs6ILu5O74qKSJ0c+4vrchDxs=