VAL_1 as written is the only MATH routine that requires BASIC ZP to be intact

[mooinglemur]

This issue is being opened so that we can revisit later if we want to refactor VAL_1 so that it doesn't use any BASIC ZP.

[irmen]

Here's some more info on why the current implementation is a bit problematic: commanderx16/x16-rom#242 (comment)

In this blog post https://www.pagetable.com/?p=774 there's a link to the M6502.MAC source file that seems to include some form of float input code. Search for "FLOATING POINT INPUT ROUTINE". You can see it calls CHRGET, which is the main problem. So this code can't be used but at least it is commented as to what it is doing