Adds data model for macro definitions

src/main/java/com/amazon/ion/impl/macro/Macro.kt

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+package com.amazon.ion.impl.macro
+
+import java.math.BigDecimal
+
+/**
+ * Marker interface for Macros
+ */
+sealed interface Macro
+
+/**
+ * Represents a template macro. A template macro is defined by a name, a signature, and a list of template expressions.
+ */
+data class TemplateMacro(val name: String, val f: BigDecimal, val signature: MacroSignature, val body: List<TemplateExpression>): Macro
+
+/**
+ * Macros that are built in, rather than being defined by a template.
+ */
+enum class SystemMacro: Macro {
+    Stream, // A stream is technically not a macro, but we can implement it as a macro that is the identity function.
+    Annotate,
+    MakeString,
+    // TODO: Other system macros
+}

src/main/java/com/amazon/ion/impl/macro/MacroRef.kt

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+package com.amazon.ion.impl.macro
+
+/**
+ * A reference to a particular macro, either by name or by template id.
+ */
+sealed interface MacroRef {
+    @JvmInline value class ByName(val name: String): MacroRef
+    @JvmInline value class ById(val id: Long): MacroRef
+}

src/main/java/com/amazon/ion/impl/macro/MacroSignature.kt

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+package com.amazon.ion.impl.macro
+
+/**
+ * Represents the signature of a macro.
+ */
+@JvmInline value class MacroSignature(val parameters: List<Parameter>) {
+
+    data class Parameter(val variableName: String, val type: Encoding, val grouped: Boolean)
+
+    enum class Encoding(val ionTextName: String?) {
+        Tagged(null),
+        Any("any"),
+        Int8("int8"),
+        // TODO: List all of the possible tagless encodings
+    }
+}

src/main/java/com/amazon/ion/impl/macro/TemplateExpression.kt

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+package com.amazon.ion.impl.macro
+
+import com.amazon.ion.SymbolToken
+import com.amazon.ion.impl.macro.ionelement.api.IonElement
+
+/**
+ * Represents an expression in the body of a template.
+ */
+sealed interface TemplateExpression {
+    // TODO: Special Forms (if_void, for, ...)?
+
+    /**
+     * A value that is taken literally. I.e. there can be no variable substitutions or macros inside here. Usually,
+     * these will just be scalars, but it is possible for it to be a container type.
+     *
+     * We cannot use [`IonValue`](com.amazon.ion.IonValue) for this because `IonValue` requires references to parent
+     * containers and to an IonSystem which makes it impractical for reading and writing macros definitions.
+     * For now, we are using [IonElement], but that is not a good permanent solution. It is copy/pasted into this repo
+     * to avoid a circular dependency, and the dependencies are shaded, so it is not suitable for a public API.
+     */
+    @JvmInline value class LiteralValue(val value: IonElement): TemplateExpression
+
+    /**
+     * An Ion List that could contain variables or macro invocations.
+     */
+    data class ListValue(val annotations: List<SymbolToken>, val expressionRange: IntRange): TemplateExpression
+
+    /**
+     * An Ion SExp that could contain variables or macro invocations.
+     */
+    data class SExpValue(val annotations: List<SymbolToken>, val expressionRange: IntRange): TemplateExpression
+
+    /**
+     * An Ion Struct that could contain variables or macro invocations.
+     */
+    data class StructValue(val annotations: List<SymbolToken>, val expressionRange: IntRange, val templateStructIndex: Map<SymbolToken, IntArray>)
+
+    /**
+     * A reference to a variable that needs to be expanded.
+     */
+    @JvmInline value class Variable(val signatureIndex: Int): TemplateExpression
+
+    /**
+     * A macro invocation that needs to be expanded.
+     */
+    data class MacroInvocation(val macro: MacroRef, val argumentExpressionsRange: IntRange): TemplateExpression
+}