Instrument ram cleanup

[n1LS]

• Optimizes padding in Variable and moves the two string pointers into a union
• Migrates from etl::list to etl::array for the Instrument's Variables() listing.

Figures below are for the pico-build.

Memory region         Used Size  Region Size  %age Used
           FLASH:      716308 B         2 MB     34.16%
             RAM:      233688 B       256 KB     89.14%

Changed Variable containers from etl::list to etl::array

Memory region         Used Size  Region Size  %age Used
           FLASH:      707224 B         2 MB     33.72%
             RAM:      227028 B       256 KB     86.60%

--> 6.660 Bytes (2.54%)

Changes in Variable (padding, union)

Memory region         Used Size  Region Size  %age Used
           FLASH:      707080 B         2 MB     33.72%
             RAM:      222484 B       256 KB     84.87%

--> 4.544 Bytes (1.73%)

==> 11.204 Bytes (4.27%)

[democloid]

If you want to change vector for array that is probably ok, but I don't see why we would change everything so radically when etl::array still offers insert methods, iterators, etc.

We can probably also also put the variable_ initialization in the constructor initializer list if you'd like

[maks]

@n1LS these are 2 completely orthogonal changes here, can you please split them into separate PRs so we can review each on its own merits vs the current all or nothing of this PR.

[n1LS]

@n1LS these are 2 completely orthogonal changes here, can you please split them into separate PRs so we can review each on its own merits vs the current all or nothing of this PR.

Sure:
#1401 Variable footprint
#1402 List -> Array

If you want to change vector for array that is probably ok, but I don't see why we would change everything so radically when etl::array still offers insert methods, iterators, etc.

etl::array doesn't have insert/push. Using etl::array_view allows the whole thing to be a bit more concise.

We can probably also also put the variable_ initialization in the constructor initializer list if you'd like

That will actually clean things up a bit, good idea.

Both of those changes are in the separated PRs.