Fix miscompilation of non-ASCII Latin-1 strings.

Fix miscompilation of non-ASCII Latin-1 strings.

The bug was introduced dad6817f ("Use the R6RS I/O API in
`write-bytecode'.").

* module/language/assembly/compile-bytecode.scm
  (write-bytecode)[write-string]: Rename to...
  [write-latin1-string]: ... this.  Add the `write-loader-len' call.
  Write each character individually instead of using `string->utf8'.
  [write-loader]: Remove.

* test-suite/tests/asm-to-bytecode.test ("compiler")[load-string "æ"]:
  New test.