Lexer_getNumber faster number parsing

In Lexer_getNumber, numbers were calculated by getting each of their numbers as charcodes (integers), converting them to one character strings, putting all the strings in an array, eventually merging the strings, an finally converting the strings back to a number.

In this newer version, there is no type conversion, the number is calculated from the char codes of its digits, with only basic arithmetic operations. (+ * / and Math.pow for numbers in scientific notation).