Allow custom types to be used for Pos, EndPos and Token.

Specifically, these fields can now be of any type that is convertible
from the internal Participle types.

README.md

@@ -596,13 +596,15 @@ There are a few areas where Participle can provide useful feedback to users of y
 	1. Of type [Error](https://pkg.go.dev/github.com/alecthomas/participle/v2#Error). This will contain positional information where available.
 	2. May either be [ParseError](https://pkg.go.dev/github.com/alecthomas/participle/v2#ParseError) or [lexer.Error](https://pkg.go.dev/github.com/alecthomas/participle/v2/lexer#Error)
 2. Participle will make a best effort to return as much of the AST up to the error location as possible.
-3. Any node in the AST containing a field `Pos lexer.Position` will be automatically
+3. Any node in the AST containing a field `Pos lexer.Position` [^1] will be automatically
    populated from the nearest matching token.
-4. Any node in the AST containing a field `EndPos lexer.Position` will be
+4. Any node in the AST containing a field `EndPos lexer.Position` [^1] will be
    automatically populated from the token at the end of the node.
-5. Any node in the AST containing a field `Tokens []lexer.Token` will be automatically
+5. Any node in the AST containing a field `Tokens []lexer.Token` [^1] will be automatically
    populated with _all_ tokens captured by the node, _including_ elided tokens.
 
+[^1]: Either the concrete type or a type convertible to it, allowing user defined types to be used.
+
 These related pieces of information can be combined to provide fairly comprehensive error reporting.
 
 ## Comments

nodes.go

@@ -131,11 +131,11 @@ func newStrct(typ reflect.Type) *strct {
 		usages: 1,
 	}
 	field, ok := typ.FieldByName("Pos")
-	if ok && field.Type == positionType {
+	if ok && positionType.ConvertibleTo(field.Type) {
 		s.posFieldIndex = field.Index
 	}
 	field, ok = typ.FieldByName("EndPos")
-	if ok && field.Type == positionType {
+	if ok && positionType.ConvertibleTo(field.Type) {
 		s.endPosFieldIndex = field.Index
 	}
 	field, ok = typ.FieldByName("Tokens")
@@ -172,14 +172,16 @@ func (s *strct) maybeInjectStartToken(token *lexer.Token, v reflect.Value) {
 	if s.posFieldIndex == nil {
 		return
 	}
-	v.FieldByIndex(s.posFieldIndex).Set(reflect.ValueOf(token.Pos))
+	f := v.FieldByIndex(s.posFieldIndex)
+	f.Set(reflect.ValueOf(token.Pos).Convert(f.Type()))
 }
 
 func (s *strct) maybeInjectEndToken(token *lexer.Token, v reflect.Value) {
 	if s.endPosFieldIndex == nil {
 		return
 	}
-	v.FieldByIndex(s.endPosFieldIndex).Set(reflect.ValueOf(token.Pos))
+	f := v.FieldByIndex(s.endPosFieldIndex)
+	f.Set(reflect.ValueOf(token.Pos).Convert(f.Type()))
 }
 
 func (s *strct) maybeInjectTokens(tokens []lexer.Token, v reflect.Value) {