Add strict mode to varname

[breuleux]

This adds a strict argument to varname, register and Wrapper that requires a direct assignment to the variable. In other words:

def function(strict):
    return varname(strict=strict)

func = function(True)     # OK, direct assignment, func == 'func'

func = [function(True)]   # Not a direct assignment, raises ImproperUseError
func = [function(False)]  # OK, func == ['func']

func = function(False), function(False)   # OK, func = ('func', 'func')

Insofar that the plan is for strict to default to True starting in 0.8.0, when strict is not given and the assignment is not direct, the old behavior is preserved but displays a deprecation warning.

Supersedes #56