Add a string operand cache

String operands used to be simple -- each line had 62 characters
plus two hard-coded non-ASCII delimiters -- but now we're mixing
character and hex data, so we can't use simple math to tell where
the lines will break.  We want to render them and keep the result
around until some dependency changes, e.g. different delimiters
or a change to the pseudo-op table.

Also, cleaned up LineListGen a little.  It had some methods that
were declared static because they were expected to be shared, but
that never happened.

Also, fixed a bug in GatherEntityCounts where multi-line items were
being scanned multiple times.

SourceGen/SourceGen.csproj

@@ -76,6 +76,7 @@
     <Compile Include="AsmGen\IAssembler.cs" />
     <Compile Include="AsmGen\IGenerator.cs" />
     <Compile Include="AsmGen\LabelLocalizer.cs" />
+    <Compile Include="FormattedOperandCache.cs" />
     <Compile Include="Tests\GenTest.cs" />
     <Compile Include="Tests\ProgressMessage.cs" />
     <Compile Include="Tests\WpfGui\GenTestRunner.xaml.cs">