- add instruction `pop_count` (efd90c65)
    `pop_count` is a u32 instruction that computes the hamming weight of
    its argument. The argument is the stack's top-most element, which is
    replaced with the result. As for all u32 instructions, the VM
    crashes if the argument is not a u32.
- parse instructions `dup` and `swap` as taking arguments (4eecac2b)
    Require `dup` and `swap` instructions to be written as `dup n`
    (respectively `swap n`) instead of `dupn` (respectively `swapn`).
    Using the following regular expression in a search-and-replace
    should greatly simplify fixing all breaking TASM:
    ```
    search for:   (dup|swap)([0-9][0-9]?)
    replace with: $1 $2
    ```
- enforce labels to start with an alphabetic character or `_` (5a5e6bad)
- remove method `simulate_no_input` (089af774)
- rename `run` to `debug`, introduce new `run` without debug
    capabilities (8bd880ff)