* module/system/base/compile.scm (next-pass): Invoke the language's
compiler chooser if there is more than one compiler.
(compute-compiler): Ensure from and to are languages.
* module/system/base/language.scm (<language>): Add compiler-chooser
field.
* module/language/brainfuck/spec.scm (choose-compiler, brainfuck):
Define a compiler chooser.