Update x->alpha for destination X

When the destination is register X:

• if there is a character code in X, then x->alpha X returns in X the corresponding character, replacing the original character code
• if there is a real matrix in X, then x->alpha X convert each matrix element to a character and returns in X a string concatenating all these characters, replacing the original matrix
• if there is a string in X, then x->alpha X concatenate this string to itself and returns the resulting string in X replacing the original string

In all cases the original content of X is saved in L.

Closes #1417 (closed)