Tuple-destructure into existing variables

[mr-zwets]

It would be clean if cashscript allowed to redefine variables in tuples, for example:

bytes variableA = 0x00;
bytes variableB = 0x00;
if(condition){
  variableA,variableB = bytesequence.split(index); 
}

Currently the workaround is

bytes variableA = 0x00;
bytes variableB = 0x00;
if(condition){
  variableA = bytesequence.split(index)[0]; 
  variableB = bytesequence.split(index)[1];
}

[mr-zwets]

renamed the issue "Tuple-destructure into existing variables" which is more clear/correct than the prior"Can't redefine variables in tuples"

with multi-return user functions (see #153) tuple-destructure will become much more common, and with it this language item

[rkalis]

Now that we have multi-return values (which are internally the same type as Tuple from .split()), we should consider to do 1 (or none) of the following:

• Allow [i] indexing of multiple return type tuples
• Disallow [0]/[1] indexing of .split() tuples