using only loop, param expansion and printf to convert string to integer

[ephiguxta]

Hello, it's been a while :)

This PR proposes using only printf to convert string to integer. For example, if we try to convert chars that the ASCII Table doesn't include, we'll get a negative number:

$ bh_str2dec "olá mundo"
111 108 -61 -95 32 109 117 110 100 111

But, with printf we get the positive value beyond the scope of ASCII:

$ bh_str2dec "olá mundo"
111 108 225 109 117 110 100 111