Use hubpack instead of ssmarshal for kipc

[mkeeter]

As part of Project Hubpack Everywhere, this is low-hanging fruit.

Kernel tests still pass, and I can use jefe as usual.

However, this increases flash usage significantly, at least on test/tests-stm32h7/app-h753.toml

Before (master, 7cace18d):

flash:
      ADDRESS  | PROGRAM |  USED |  SIZE | LIMIT
    0x08000000 | kernel | 32768 | 32768 | (fixed)
    0x08008000 | runner | 13456 | 16384 | 32768
    0x0800c000 | assist | 10240 | 16384 | 32768
    0x08010000 | suite  | 40428 | 65536 | 65536
    0x08020000 | idol   |  3700 |  4096 | 4096
    0x08021000 | idle   |    84 |   128 | 256
    0x08021080 | --end-- |       |       |

ram:
      ADDRESS  | PROGRAM |  USED |  SIZE | LIMIT
    0x20000000 | kernel |  2100 |  4096 | (fixed)
    0x20001000 | runner |  2184 |  4096 | 4096
    0x20002000 | suite  |  2176 |  4096 | 4096
    0x20003000 | assist |  2176 |  4096 | 4096
    0x20004000 | idol   |  1024 |  1024 | 1024
    0x20004400 | idle   |   256 |   256 | 256
    0x20004500 | --end-- |       |       |

After (44cb16f0):

flash:
      ADDRESS  | PROGRAM |  USED |  SIZE | LIMIT
    0x08000000 | kernel | 32768 | 32768 | (fixed)
    0x08008000 | runner | 20064 | 32768 | 32768
    0x08010000 | suite  | 40608 | 65536 | 65536
    0x08020000 | assist | 17292 | 32768 | 32768
    0x08028000 | idol   |  3732 |  4096 | 4096
    0x08029000 | idle   |    84 |   128 | 256
    0x08029080 | --end-- |       |       |

ram:
      ADDRESS  | PROGRAM |  USED |  SIZE | LIMIT
    0x20000000 | kernel |  2100 |  4096 | (fixed)
    0x20001000 | runner |  2184 |  4096 | 4096
    0x20002000 | suite  |  2176 |  4096 | 4096
    0x20003000 | assist |  2176 |  4096 | 4096
    0x20004000 | idol   |  1024 |  1024 | 1024
    0x20004400 | idle   |   256 |   256 | 256
    0x20004500 | --end-- |       |       |

runner increased by 6.6K, and assist by 7K.