Expand Signal api

[kaqu]

Signal api allows now only basic transformations and handlers. There are some additional ones that will be nice to have. Some possible enhancements:

• handler on any token - no matter if error or value.
• zip on signals
• recover from error to value
• catch errors without further propagation
• catch values without further propagation
• error mapping - connected with strongly typed errors enhancement #21 and #23

[kaqu]

partly done in 2.1.0