[mbrunnen]

Fixes #25.

parse_unary now parses the operand of a unary operator as another unary
expression, so - -x, ~~x and not not b parse. KerML 1.0 gives the
operand as an ArgumentMember, so any expression
(8.2.5.8.1 Operator Expressions).
The Pilot's KerMLExpressions.xtext narrows it to ExtentExpression.

How to test

$ cargo test -p sysmlv2-parser --test integration unary_operators_nest
$ printf 'package P { attribute a = - -x; }\n' | sysmlv2 parse -

Look closely at

• The recursion goes through descend, so the nesting bound still applies
  to a long run of operators.
• Precedence does not change: - -2 ** 3 parses as (-(-2)) ** 3, as
  -2 ** 3 parses as (-2) ** 3.
• sysmlv2 fmt prints -(-x) as --x. That text now parses again, so
  fmt no longer refuses such a file. No baseline changes.
• The CHANGELOG.md entry is under a new Unreleased heading. Move or drop
  it as you prefer.