Static valueOf(String) for unsigned types

[bjakke]

The functionality is added in #79, however each method is named per the type.

In the jdk types Byte/Short/Integer/Long there is a valueOf(String) and valueOf(String,int) versions of those methods in addition to the type specific name, therefore the stack unsigned types could have them as well. This would making code operating on all of those types more generic e.g. with reflection as the method name is the same (although BigDecimal does not have that).