Scope: Keep value in a local variable.
src: Remove trailing white space.
Line: Make method private.
LineBuilder: Use empty list from Collections instead of a constant member.
EvaluationException: Have constructors like AssemblyException.
If: Improve expansion to match when other directives do it.

It used to register and expand later than usual in resolve / getBytes, because
we don’t want to evaluate the condition sooner in case $ is used in it.
However we now give the then and else sources their own scope, and register
symbols in the containing scope with the condition embedded in them.

This makes ifs work better inside macros.
Scope: Type checks never throw so try/catch is only needed for the evaluation.

Since we introduced the error type.
ErrorLiteral: Introduce an error type.

This represents bottom from lambda calculus, a computation which never completes
successfully. It is a member of any type, and throws an exception on evaluation.

This allows some expressions to evaluate without throwing errors unnecessarily.
For example, scope lookups in a ternary which never evaluates to take the path
which causes the error.
Line: Also copy directives with sources inside.

It would cause symbol / address redefinition and not found errors when used in
repeat blocks (which copy). Note there is some duplication of logic in
SourceBuilder and the copy method with regards to which scope is used.
Scope: Include all scope integer values (like equs) in symbol file output.
Scope: Include macro definitions in symbol file output.
test: Improve exception assertions to include context checks etc.

Adds helper methods to Parser, SourceFileSpan and SymbolNotFoundException.

In the process we also improve a couple of InstructionTests by splitting the two
throwing parses into two assertions, and the ParserTests for character literals
which were syntax erroring on the wrong thing.
TestBase: Introduce base class for tests to share code.
SymbolsTest: Add test for symbol file generation.
SourceTest: Simplify string joining.
SourceTest: Improve some section tests by checking return values.

Rather than only checking for no errors.
MacroInstruction/Irp/Rept/Proc: Use methods on Source copy.
Source/Line: Replace rescoping copy constructor with an explicit copy method.

Like expressions have. The action is more explicit this way.
Source: Extract expand method.
Source: Remove unused return values.
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