unicode-combine: remove utf8proc dependency

We only used utf8proc to try to pre-compose a glyph from a base and
combining character.

We can do this ourselves by using a pre-compiled table of valid
pre-compositions. This table isn't _that_ big, and binary searching it
is fast.

That is, for a very small amount of code, and not too much extra RO
data, we can get rid of the utf8proc dependency.

scripts/generate-unicode-precompose.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/sh
+
+cut - -d ";" -f 1,6 |
+    grep ";[0-9,A-F]" | grep " " |
+    sed -e "s/ /, 0x/;s/^/{ 0x/;s/;/, 0x/;s/$/},/" |
+    sed -e "s,0x\(....\)\([^0-9A-Fa-f]\),0x0\1\2,g" |
+    (sort -k 3 || sort +2) |
+    sed -e "s,0x0\(...[0-9A-Fa-f]\),0x\1,g"