Make $hp point to the first available byte, instead of the one before…

… it (#466)

See FuelLabs/fuel-vm#377

src/vm/index.md

@@ -186,9 +186,9 @@ Whenever memory is written to (i.e. with [`SB`](./instruction_set.md#sb-store-by
 If the context is external, the owned memory range is:
 
 1. `[$ssp, $sp)`: the writable stack area.
-1. `($hp - 1, VM_MAX_RAM - 1]`: the heap area allocated by this script or predicate.
+1. `[$hp, VM_MAX_RAM)`: the heap area allocated by this script or predicate.
 
 If the context is internal, the owned memory range for a call frame is:
 
 1. `[$ssp, $sp)`: the writable stack area of the call frame.
-1. `($hp - 1, $fp->$hp - 1]`: the heap area allocated by this call frame.
+1. `[$hp, $fp->$hp)`: the heap area allocated by this call frame.

src/vm/instruction_set.md

@@ -804,8 +804,8 @@ All these instructions advance the program counter `$pc` by `4` after performing
 
 Panic if:
 
-- `$hp - $rA - 1` underflows
-- `$hp - $rA - 1 < $sp`
+- `$hp - $rA` underflows
+- `$hp - $rA < $sp`
 
 ### CFEI: Extend call frame immediate
 
@@ -820,7 +820,7 @@ Panic if:
 Panic if:
 
 - `$sp + imm` overflows
-- `$sp + imm > $hp - 1`
+- `$sp + imm > $hp`
 
 ### CFSI: Shrink call frame immediate
 
@@ -1224,7 +1224,7 @@ Panic if:
 - `$rA + 32` overflows
 - `$ssp + $rC > VM_MAX_RAM`
 - `$rA + 32 > VM_MAX_RAM`
-- `$ssp + $rC > $hp - 1`
+- `$ssp + $rC >= $hp`
 - `$rC > CONTRACT_MAX_SIZE`
 - `$rC > MEM_MAX_ACCESS_SIZE`
 - Contract with ID `MEM[$rA, 32]` is not in `tx.inputs`