This adds a generic non-recursive implementation of Tarjan's linear time
SCC algorithm that produces components in topological order. It can be
instantiated to work directly on any graph representation for which the
enumerate_nodes and enumerate_successors interface can be implemented.

When implementing custom log_... functions or custom overloads for the
core log functions like log_signal it is necessary to return `char *`
that are valid long enough.

The log_... functions implemented in log.cc use either `log_id_cache` or
`string_buf` which both are cleared on log_pop.

This commit adds a public `log_str` function which stores its argument
in the `log_id_cache` and returns the stored copy, such that custom
log functions outside of log.cc can also create strings that remain
valid until the next `log_pop`.

This is already supported by `SigSpec` and since both `SigChunk` and
`SigSpec` implement `extract` which is the multi-bit variant of this,
there is no good reason for `SigChunk` to not support
`SigBit operator[](int offset)`.

It adds `DriveBit`, `DriveChunk` and `DriveSpec` types which are similar
to `SigBit`, `SigChunk` and `SigSpec` but can also directly represent
cell ports, undriven bits and multiple drivers. For indexing an RTLIL
module and for querying signal drivers it comes with a `DriverMap` type
which is somewhat similar to a `SigMap` but is guaranteed to produce
signal drivers as returned representatives.

A `DriverMap` can also optionally preserve connections via intermediate
wires (e.g. querying the driver of a cell input port will return a
connected intermediate wire, querying the driver of that wire will
return the cell output port that's driving the wire).

…rivers

…unctional backend

…se it

…on with yosys sim

…c++ functional backend

…ional backend

…1) nor (pop 1)

…ructor (leads to weird constructor overloading resolution issues)

…tidy them up/document them

…sitor. remove extraneous widths arguments from visitors.

…unctionalir.cc

…ackend, implement $fa,$lcu,$alu

…d comments for memory edgecases

…are seen, dont bother putting them in functionalir

…st file

… backend

…ly again

…nd current_state (more consistent with other methods)

…states nodes, other nodes get deleted

…alir.h to functional.h, rename functional.h to compute_graph.h

… below other definitions

…the terminology consistent

Also unique_name can take field_name directly.

internal_stats: init, report current memory consumption on linux and mac

Note sure if this is the best way to do it, but it works?

`-W --keep-going` will build everything but raise an error on any warnings.  Should mean that github actions will correctly flag a failure when attempting to build docs.  Including this current commit until the `internal_stats` command gets a help message.