Fix possible segfaults after GC.compact

This adds several rb_gc_mark calls for VALUEs that were only implicit marked before.
This is because ruby-2.7 adds a second meaning to rb_gc_mark().
It not only marks a VALUE as in use, but also marks the VALUE as unmoveable, so that it's pinned to one memory address.

In ruby < 2.7 is was sufficient to mark objects that otherwise would freed by GC.
So there was no need to mark objects that were implicit in use, like self references.
Strating with ruby-2.7 these objects must be marked as well.
Alternatively C extentions can use compatation callbacks to update VALUE references.

I verified that this patch is effective to fix these issues by placing several instances of the following line into specs:
  GC.verify_compaction_references(toward: :empty, double_heap: true)

Unfortunately GC.verify_compaction_references is not suitable to be added regulary to the specs, because every instance of this call doubles the process memory in use.

See also GC.compact discussion: https://bugs.ruby-lang.org/issues/15626#Known-Issue

Fixes #327
Fixes #328

ext/pg.h

@@ -304,6 +304,7 @@ VALUE pg_obj_to_i                                      _(( VALUE ));
 VALUE pg_tmbc_allocate                                 _(( void ));
 void pg_coder_init_encoder                             _(( VALUE ));
 void pg_coder_init_decoder                             _(( VALUE ));
+void pg_coder_mark                                     _(( t_pg_coder * ));
 char *pg_rb_str_ensure_capa                            _(( VALUE, long, char *, char ** ));
 
 #define PG_RB_STR_ENSURE_CAPA( str, expand_len, curr_ptr, end_ptr ) \

ext/pg_coder.c

@@ -61,11 +61,23 @@ pg_coder_init_decoder( VALUE self )
 	rb_iv_set( self, "@name", Qnil );
 }
 
+void
+pg_coder_mark(t_pg_coder *this)
+{
+	rb_gc_mark(this->coder_obj);
+}
+
+static void
+pg_composite_coder_mark(t_pg_composite_coder *this)
+{
+	pg_coder_mark(&this->comp);
+}
+
 static VALUE
 pg_simple_encoder_allocate( VALUE klass )
 {
 	t_pg_coder *this;
-	VALUE self = Data_Make_Struct( klass, t_pg_coder, NULL, -1, this );
+	VALUE self = Data_Make_Struct( klass, t_pg_coder, pg_coder_mark, -1, this );
 	pg_coder_init_encoder( self );
 	return self;
 }
@@ -74,7 +86,7 @@ static VALUE
 pg_composite_encoder_allocate( VALUE klass )
 {
 	t_pg_composite_coder *this;
-	VALUE self = Data_Make_Struct( klass, t_pg_composite_coder, NULL, -1, this );
+	VALUE self = Data_Make_Struct( klass, t_pg_composite_coder, pg_composite_coder_mark, -1, this );
 	pg_coder_init_encoder( self );
 	this->elem = NULL;
 	this->needs_quotation = 1;
@@ -87,7 +99,7 @@ static VALUE
 pg_simple_decoder_allocate( VALUE klass )
 {
 	t_pg_coder *this;
-	VALUE self = Data_Make_Struct( klass, t_pg_coder, NULL, -1, this );
+	VALUE self = Data_Make_Struct( klass, t_pg_coder, pg_coder_mark, -1, this );
 	pg_coder_init_decoder( self );
 	return self;
 }
@@ -96,7 +108,7 @@ static VALUE
 pg_composite_decoder_allocate( VALUE klass )
 {
 	t_pg_composite_coder *this;
-	VALUE self = Data_Make_Struct( klass, t_pg_composite_coder, NULL, -1, this );
+	VALUE self = Data_Make_Struct( klass, t_pg_composite_coder, pg_composite_coder_mark, -1, this );
 	pg_coder_init_decoder( self );
 	this->elem = NULL;
 	this->needs_quotation = 1;

ext/pg_copy_coder.c

@@ -23,6 +23,7 @@ typedef struct {
 static void
 pg_copycoder_mark( t_pg_copycoder *this )
 {
+	pg_coder_mark(&this->comp);
 	rb_gc_mark(this->typemap);
 	rb_gc_mark(this->null_string);
 }

ext/pg_record_coder.c

@@ -18,6 +18,7 @@ typedef struct {
 static void
 pg_recordcoder_mark( t_pg_recordcoder *this )
 {
+	pg_coder_mark(&this->comp);
 	rb_gc_mark(this->typemap);
 }
 

ext/pg_type_map_by_class.c

@@ -126,7 +126,11 @@ pg_tmbk_mark( t_tmbk *this )
 {
 	rb_gc_mark(this->typemap.default_typemap);
 	rb_gc_mark(this->klass_to_coder);
-	/* All coders are in the Hash, so no need to mark the cache. */
+	rb_gc_mark(this->self);
+	/* Clear the cache, to be safe from changes of klass VALUE by GC.compact.
+	 * TODO: Move cache clearing to compactation callback provided by Ruby-2.7+.
+	 */
+	memset(&this->cache_row, 0, sizeof(this->cache_row));
 }
 
 static VALUE