updatePosChar
Update a source position given a character. If the character is a newline ('\n') or carriage return ('\r') the line number is incremented by 1. If the character is a tab ('t') the column number is incremented to the nearest 8'th column, ie. column + 8 - ((column-1) `mod` 8). In all other cases, the column is incremented by 1.