`walk-parts' walked the whole `var' form, and a `fn' type's parameter
list is shaped like a call: `(fn ((c int)) int)' came out as
`int (*fp)(c int)'. A closure literal had no type, so calling one in
place missed the rewrite; sextest fed sexc symbols Sex cannot read.
sextest resolves #+ and #- itself -- it reads the program and prints
what survives to sexc -- so a flag spelling it does not recognise
decides which branch gets compiled
1. Look for `compilation', not `compile' among the source
2. Read the file with provided --features from the (compilation ...)
form, then handle resulting file to sexc
