Various checker enhancements, better Error messages

[dmlockhart]

Throw an error if:

• a signal is written in multiple behavioral blocks (not valid verilog)
  [ is this related to this? (Add checking to detect multiple identical assignments/multiple assignments to the same signal. #127) ]
• a signal is written as .next in one block and .value as another
• a signal is structurally written to by a connection and written to in a behavioral block
• a signal is structurally written to by multiple connections
• the .value/.next of an OutPort is written from outside a module
• the .value/.next of an InPort is written from inside a module

Throw a better error message when:

• s.connect( s.a, s.b ) fails because s.a is a port list and should be s.a[N]. Currently get:

  AttributeError: 'list' object has no attribute 'nbits'