Refactor PtcInfo (#2625)

* Refactor `PtcInfo`

This change reduces the coupling of `PtcInfo` by moving relocation
tracking to the backend. `RelocEntry`s remains as `RelocEntry`s through
out the pipeline until it actually needs to be written to the PTC
streams. Keeping this representation makes inspecting and manipulating
relocations after compilations less painful. This is something I needed
to do to patch relocations to 0 to diff dumps.

Contributes to #1125.

* Turn `Symbol` & `RelocInfo` into readonly structs

* Add documentation to `CompiledFunction`

* Remove `Compiler.Compile<T>`

Remove `Compiler.Compile<T>` and replace it by `Map<T>` of the
`CompiledFunction` returned.

ARMeilleure/CodeGen/Linking/Symbol.cs

@@ -0,0 +1,100 @@
+using System;
+
+namespace ARMeilleure.CodeGen.Linking
+{
+    /// <summary>
+    /// Represents a symbol.
+    /// </summary>
+    readonly struct Symbol
+    {
+        private readonly ulong _value;
+
+        /// <summary>
+        /// Gets the <see cref="SymbolType"/> of the <see cref="Symbol"/>.
+        /// </summary>
+        public SymbolType Type { get; }
+
+        /// <summary>
+        /// Gets the value of the <see cref="Symbol"/>.
+        /// </summary>
+        /// <exception cref="InvalidOperationException"><see cref="Type"/> is <see cref="SymbolType.None"/></exception>
+        public ulong Value
+        {
+            get
+            {
+                if (Type == SymbolType.None)
+                {
+                    ThrowSymbolNone();
+                }
+
+                return _value;
+            }
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Symbol"/> structure with the specified <see cref="SymbolType"/> and value.
+        /// </summary>
+        /// <param name="type">Type of symbol</param>
+        /// <param name="value">Value of symbol</param>
+        public Symbol(SymbolType type, ulong value)
+        {
+            (Type, _value) = (type, value);
+        }
+
+        /// <summary>
+        /// Determines if the specified <see cref="Symbol"/> instances are equal.
+        /// </summary>
+        /// <param name="a">First instance</param>
+        /// <param name="b">Second instance</param>
+        /// <returns><see langword="true"/> if equal; otherwise <see langword="false"/></returns>
+        public static bool operator ==(Symbol a, Symbol b)
+        {
+            return a.Equals(b);
+        }
+
+        /// <summary>
+        /// Determines if the specified <see cref="Symbol"/> instances are not equal.
+        /// </summary>
+        /// <param name="a">First instance</param>
+        /// <param name="b">Second instance</param>
+        /// <returns><see langword="true"/> if not equal; otherwise <see langword="false"/></returns>
+        /// <inheritdoc/>
+        public static bool operator !=(Symbol a, Symbol b)
+        {
+            return !(a == b);
+        }
+
+        /// <summary>
+        /// Determines if the specified <see cref="Symbol"/> is equal to this <see cref="Symbol"/> instance.
+        /// </summary>
+        /// <param name="other">Other <see cref="Symbol"/> instance</param>
+        /// <returns><see langword="true"/> if equal; otherwise <see langword="false"/></returns>
+        public bool Equals(Symbol other)
+        {
+            return other.Type == Type && other._value == _value;
+        }
+
+        /// <inheritdoc/>
+        public override bool Equals(object obj)
+        {
+            return obj is Symbol sym && Equals(sym);
+        }
+
+        /// <inheritdoc/>
+        public override int GetHashCode()
+        {
+            return HashCode.Combine(Type, _value);
+        }
+
+        /// <inheritdoc/>
+        public override string ToString()
+        {
+            return $"{Type}:{_value}";
+        }
+
+        private static void ThrowSymbolNone()
+        {
+            throw new InvalidOperationException("Symbol refers to nothing.");
+        }
+    }
+}