Revisit simple parameter expansion

[pombredanne]

The simple parameters such as $foo or ${bar} are expanded by simple replacement which may not be entirely correct

In expand_simple() we need to validate if a plain replace is really what is supposed to happen in particular simple expansion may need to happen on tokens rather than on the whole string.

For instance, with foo=bar, abc$fooBAR does not expand in Bash

Also foo=BUZ;foobar=BAZ;echo "$foobar" which means the longest parameter name is matched first and we do not apply this correctly.