Support textual format in WASM backends

[athas]

When the input data for a test program is in a file, the file will be passed directly to the restore server command. This means the contents might be in the textual format, which we will therefore need to support. We don't need to support the crazy mixed textual/binary format (but if it's not too difficult it doesn't hurt).

[philass]

I'll get working on this one