munge percent signs in identifiers

so AIDS model will work
make `user-message` a no-op for now

needed to get Segregation running
fix buggy `patch-at`, `turtles-at` primitives

they didn't work for turtles since they measured from the patch
center instead of from the exact location of the turtle
new prims: `subtract-headings`, `dx`, `dy`
bunches more models pass now!
fix turtle-creation prims to handle negative numbers
add __boom primitive
add xor primitive
list/string prims: member, position, remove, et al

also remove-item, replace-item, sublist, sentence, is-string?,
substring
delete trailing whitespace
add a few dummy mouse and plot prims

so models that make incidental use of them can be used
in the docking tests
add patchesWithLabels support

this is needed for Traffic Grid to pass the docking tests
more prims: exp, variance, base-colors
more models work now!

Traffic Grid, Peppered Moths, Fur, Ising, GenDrift P global, GenDrift
P local, GenDrift T reproduce
fix _taskvariable to work inside blocks
GenDrift T interact works now
supply some missing semicolons
Ethnocentrism works now
make labels work properly (always mirrored as strings)

needs NetLogo dependency bump since the main fix was in that repo
bugfix to patch labels (count patchesWithLabels properly)