Fixes#844
This PR adds a new machine to the STD: `WriteOnceMemory`. This can be
used in our RISC-V machine for bootloader inputs (#1203).
Most of the issues mentioned in the issue were fixed in the meantime or
had a simple workaround (like defining `let LATCH = 1`). The only
remaining issues were in the machine detection, which I fixed here.
I also re-factor two existing tests.
Allows braced blocks everywhere where expressions are expected.
The statements in those blocks can be `let x` or `let x = ...`.
The former declares a new witness column, the latter just binds a local
variable.
fixes#959
With the recent changes by @pacheco, we can extract our [memory
machine](https://github.com/powdr-labs/powdr/blob/main/riscv/src/compiler.rs#L687-L841)
as a separate machine and add it to the standard library.
The result should be the same as calling the function linked above with
`with_bootloader=false`, except that the memory alignment stuff is not
inlined. For this reason, the machine is not yet used by the RISC-V
machine, but it could be after #1077 is implemented.
[This](eb320dca0c) shows the diff from
what we have in `compiler.rs`.
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