Add support for absolute qualified identifiers

[bocchino]

Allow qualified identifiers that begin with a dot. An initial dot indicates that the qualified identifier is to be resolved from the top level, regardless of where it appears. This feature provides a way to refer to symbols that are hidden by definitions in an inner scope. It is similar to an initial :: in a C++ name qualifier.

For example:

constant a = 0
module M {
  constant a = 1 # M.a hides a here
  constant b = a # Sets b to 1
  constant c = .a # Sets c to 0
  constant d = .M.a # Same as constant d = a
}

In this scheme, there are two kinds of qualified identifiers:

• Relative qualified identifiers: Each of these is a list of simple names separated by dots. It specifies a qualified name relative to the innermost scope in which the first simple name is defined. For example, when a is defined in scope M, then inside scope M, a refers to M.a. This is true regardless of whether a is defined at the top level.
• Absolute qualified identifiers: Each of these is a dot followed by a relative qualified identifier. It specifies a name relative to the top scope. For example, when a is defined at the top level, then .a refers to a. This is true even in a scope M that also defines a.