C#: Replace RewriteRule with CSharpTemplate.Rewrite()

[knutwannheden]

Motivation

The IRewriteRule / RewriteRule API was an intermediate abstraction between raw Match/Apply and a full visitor. After simplifying it down to just TryOn() and ToVisitor(), the remaining indirection doesn't carry its weight — ToVisitor() can be a static factory on CSharpTemplate directly, and TryOn() is just pat.Match() + tmpl.Apply() which is already clean enough in C#.

Examples

// Simple pattern → template (was: RewriteRule.Rewrite(pat, tmpl).ToVisitor())
return CSharpTemplate.Rewrite(
    CSharpPattern.Expression($"Console.Write({expr})"),
    CSharpTemplate.Expression($"Console.WriteLine({expr})"));

// Multiple patterns, shared template
return CSharpTemplate.Rewrite(
    [
        CSharpPattern.Expression($"Console.Write({expr})"),
        CSharpPattern.Expression($"Console.Error.Write({expr})"),
    ],
    CSharpTemplate.Expression($"Console.WriteLine({expr})"));

// Multiple (pattern, template) pairs — first match wins
return CSharpTemplate.Rewrite(
    (CSharpPattern.Expression($"{x} == null"), CSharpTemplate.Expression($"{x} is null")),
    (CSharpPattern.Expression($"{x} != null"), CSharpTemplate.Expression($"{x} is not null")));

Summary

• Remove IRewriteRule interface, RewriteRule static class, and all internal implementation classes (RewriteRuleImpl, RewriteRuleVisitor)
• Add CSharpTemplate.Rewrite() with three overloads: single pair, multiple patterns with shared template, and multiple (pattern, template) pairs
• Move CreateBlockFlattener<P>() and BlockFlattener<P> to CSharpTemplate
• Update all test recipes to use the new API or raw Match/Apply

Test plan

• All 18 RewriteRuleTests pass with the new API
• AttributeRenameViaRewriteRule in TemplateApplyTests passes
• Full template test suite (235 tests) passes
• No remaining references to IRewriteRule, RewriteRule, TryOn, or ToVisitor