RFR: Improve formatting of generated source code [v2]

[Jorn Vernee]

> As a main rule-of-thumb: we should generate a blank line before a multi-line declaration, or before a block of single line declarations (e.g. fields, or methods collapsed into a single line).
> 
> In most cases, we can just add a blank line to the start of the String template we use. In some cases that emit single-line declarations, we have a manual call to `appendBlankLine` before emitting multiple single line decls. For declarations with comments we also add the blank line manually, as there shouldn't be a blank line between the comment and declaration.
> 
> - I've also added the default constructor to every class we generate, and moved it before the layout decl in struct classes (makes a little bit more sense I think).
> - fixed the indentation and excess trailing whitespace after the class `}` of a class.
> - Fixed the way in which we add indentation to text blocks, so that we don't add trailing white space to empty lines.
> 
> <details>
> <summary>Sample output (click to unfold)</summary>
> 
> For instance for this header:
> 
> struct Foo {
>     int x;
>     struct Bar {
>         int y;
>     } bar;
> };
> 
> typedef struct Foo (*CB)(void);
> 
> 
> Foo.java:
> 
> // Generated by jextract
> 
> import java.lang.invoke.MethodHandle;
> import java.lang.invoke.MethodHandles;
> import java.lang.invoke.VarHandle;
> import java.nio.ByteOrder;
> import java.lang.foreign.*;
> import static java.lang.foreign.ValueLayout.*;
> 
> /**
>  * {@snippet lang=c :
>  * struct Foo {
>  *     int x;
>  *     struct Bar bar;
>  * };
>  * }
>  */
> public class Foo {
> 
>     Foo() {
>         // Suppresses public default constructor, ensuring non-instantiability,
>         // but allows generated subclasses in same package.
>     }
> 
>     private static final GroupLayout $LAYOUT = MemoryLayout.structLayout(
>         struct_with_fptr_h.C_INT.withName("x"),
>         Foo.Bar.$LAYOUT().withName("bar")
>     ).withName("Foo");
> 
>     public static final GroupLayout $LAYOUT() {
>         return $LAYOUT;
>     }
> 
>     private static final long x$OFFSET = 0;
> 
>     /**
>      * Getter for field:
>      * {@snippet lang=c :
>      * int x;
>      * }
>      */
>     public static int x$get(MemorySegment seg) {
>         return seg.get(struct_with_fptr_h.C_INT, x$OFFSET);
>     }
> 
>     public static int x$get(MemorySegment seg, long index) {
>         return seg.get(struct_with_fptr_h.C_INT, x$OFFSET + (index * sizeof()));
>     }
> 
>     /**
>      * Setter for field:
>      * {@snippet lang=c :
>      * int x;
>      * }
>      */
>     public static void x$set(MemorySegment seg, int x) {
>         seg.set(struct_with_fptr_...

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  review comments

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/166/files
  - new: https://git.openjdk.org/jextract/pull/166/files/2ff1d53a..14112489

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=166&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=166&range=00-01

  Stats: 14 lines in 1 file changed: 3 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/166.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/166/head:pull/166

PR: https://git.openjdk.org/jextract/pull/166