Treat percent as unit, enable conversions to percent

modules/math/partsperx.nbt

@@ -1,7 +0,0 @@
-let pct = 1e-02
-let ppm = 1e-06
-let ppb = 1e-09
-let ppt = 1e-12
-let ppq = 1e-15
-
-let percent = pct

modules/prelude.nbt

@@ -3,7 +3,6 @@ use core::dimensions
 
 use math::functions
 use math::constants
-use math::partsperx
 
 use units::si
 use units::time
@@ -14,6 +13,7 @@ use units::nautical
 use units::cgs
 use units::fff
 use units::misc
+use units::partsperx
 
 use units::currency
 use units::bit

modules/units/partsperx.nbt

@@ -0,0 +1,14 @@
+@aliases(pct)
+unit percent = 1e-02
+
+@aliases(ppm)
+unit partspermillion = 1e-06
+
+@aliases(ppb)
+unit partsperbillion = 1e-09
+
+@aliases(ppt)
+unit partspertrillion = 1e-12
+
+@aliases(ppq)
+unit partsperquadrillion = 1e-15