Add an easy way to register proxies (#321)

We already have a mechanism for selecting proxies -- SerdeTypeOptions or
SerdeMemberOptions. However, there are two downsides.

First, the proxy is meant to match the type of the member exactly. This
can get very verbose if the only thing that's interesting in the proxy
is nested inside a type substitution.

Second, there's no way to automatically use a proxy for all members in a
type.

The new `UseProxy` feature adds an easy way to pick out individual
proxies and reduce boilerplate.

Thanks to @Deficuet for the initial implementation here.

Author: agocke

src/generator/DeserializeImplGen.cs

@@ -1,13 +1,13 @@
 
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
 using System.Text;
-using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
 using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
 using static Serde.Diagnostics;
 using static Serde.SerdeImplRoslynGenerator;
@@ -200,6 +200,8 @@ private static SourceBuilder GenerateCustomDeserializeMethod(
         {
             Debug.Assert(type.TypeKind != TypeKind.Enum);
 
+            var classScopeProxyMap = ProxyMap.FromSymbol(type);
+
             var members = SymbolUtilities.GetDataMembers(type, SerdeUsage.Both);
             var typeFqn = typeSyntax.ToString();
             var assignedVarType = members.Count switch {
@@ -267,7 +269,10 @@ private static SourceBuilder GenerateCustomDeserializeMethod(
                         ? "ReadValue"
                         : "ReadBoxedValue";
                     string readValueCall;
-                    if (Proxies.TryGetExplicitWrapper(m, context, SerdeUsage.Deserialize, inProgress) is { } explicitWrap)
+
+                    var proxyContext = ProxyContext.Create(classScopeProxyMap, ProxyMap.FromSymbol(m.Symbol));
+
+                    if (Proxies.TryGetExplicitWrapper(m, context, SerdeUsage.Deserialize, inProgress, proxyContext) is { } explicitWrap)
                     {
                         readValueCall = $"{readMethodName}<{memberType}, {explicitWrap}>";
                     }
@@ -279,7 +284,7 @@ private static SourceBuilder GenerateCustomDeserializeMethod(
                     {
                         readValueCall = $"Read{primitiveName}";
                     }
-                    else if (Proxies.TryGetImplicitWrapper(m.Type, context, SerdeUsage.Deserialize, inProgress) is { Proxy: { } wrap })
+                    else if (Proxies.TryGetImplicitWrapper(m.Type, context, SerdeUsage.Deserialize, inProgress, proxyContext) is { Proxy: { } wrap })
                     {
                         if (wrap == "global::Serde.GuidProxy")
                         {