Replace global jump with catch handler implementation

When a global jump occurs, look at the catch handler table to determine where to jump.
In that case, `pc` already shows the following instruction, but since the table shows `begin_offset ... end_offset`, the comparison is done with `begin_offset < pc && pc <= end_offset`.
If there is a corresponding handler, move `pc` to `handler.target_offset` and continue running the VM.

When a global jump across `ensure` is made by `return`, `break`, `next`, `redo` and `retry`, the extended `RBreak` object saves and restores the C-level execution position.
This extended `RBreak` can have tag information, which makes it a pseudo coroutine (the "tag" mimics CRuby).

The implementation of pseudo coroutines by `RBreak` is summarized by `CHECKPOINT_RESTORE ... CHECKPOINT_MAIN ... CHECKPOINT_END` and `throw_tagged_break` / `unwind_ensure` macros.
The restart of processing is branched by `RBREAK_TAG_FOREACH(DISPATCH_CHECKPOINTS)`.

- Not only `rescue` blocks but also `ensure` blocks are now sandwiched between `OP_EXCEPT` and `OP_RAISEIF`.

- Remove the function `ecall()`.
  It is no longer necessary to re-enter the VM to perform an "ensure block".

  This will resolves #1888.

- Added instruction `OP_JUW` (Jump while UnWind).

  It jumps unconditionally like `OP_JMP`, but searches the catch handler table and executes the ensure block.
  Since it searches the catch handler table, it is much heavier than `OP_JMP`.

1 files changed, 1 insertions, 0 deletions

diff --git a/doc/opcode.md b/doc/opcode.md
index 385bf9468..abdee704f 100644
--- a/doc/opcode.md
+++ b/doc/opcode.md
@@ -62,6 +62,7 @@ sign) of operands.
 | OP_JMPIF         | BS           | if R(a) pc=b                                           |
 | OP_JMPNOT        | BS           | if !R(a) pc=b                                          |
 | OP_JMPNIL        | BS           | if R(a)==nil pc=b                                      |
+| OP_JUW           | S            | unwind_and_jump_to(a)                                  |
 | OP_EXCEPT        | B            | R(a) = exc                                             |
 | OP_RESCUE        | BB           | R(b) = R(a).isa?(R(b))                                 |
 | OP_RAISEIF       | B            | raise(R(a)) if R(a)                                    |