Move implementation files into their own folder

main.c and test.c generate binary executables so they can stay in the
main folder, but the rest can go into their own dedicated folder to
make it look nicer

build.sh

@@ -3,8 +3,8 @@
 set -xe
 
 CFLAGS="-Wall -Wextra -std=c11 -ggdb -fsanitize=address -fsanitize=undefined"
-LINK=""
-LIB="sv.c vec.c symtable.c tag.c constructor.c stream.c sys.c"
+LINK="-I."
+LIB="impl/sv.c impl/vec.c impl/symtable.c impl/tag.c impl/constructor.c impl/stream.c impl/sys.c"
 OUT="alisp.out"
 
 build() {