cmd/compile: initial rulegen rewrite

rulegen.go produces plaintext Go code directly, which was fine for a
while. However, that's started being a bottleneck for making code
generation more complex, as we can only generate code directly one line
at a time.

Some workarounds were used, like multiple layers of buffers to generate
chunks of code, to then use strings.Contains to see whether variables
need to be defined or not. However, that's error-prone, verbose, and
difficult to work with.

A better approach is to generate an intermediate syntax tree in memory,
which we can inspect and modify easily. For example, we could run a
number of "passes" on the syntax tree before writing to disk, such as
removing unused variables, simplifying logic, or moving declarations
closer to their uses.

This is the first step in that direction, without changing any of the
generated code. We didn't use go/ast directly, as it's too complex for
our needs. In particular, we only need a few kinds of simple statements,
but we do want to support arbitrary expressions. As such, define a
simple set of statement structs, and add thin layers for printer.Fprint
and ast.Inspect.

A nice side effect of this change, besides removing some buffers and
string handling, is that we can now avoid passing so many parameters
around. And, while we add over a hundred lines of code, the tricky
pieces of code are now a bit simpler to follow.

While at it, apply some cleanups, such as replacing isVariable with
token.IsIdentifier, and consistently using log.Fatalf.

Follow-up CLs will start improving the generated code, also simplifying
the rulegen code itself. I've added some TODOs for the low-hanging fruit
that I intend to work on right after.

Updates #30810.

Change-Id: Ic371c192b29c85dfc4a001be7fbcbeec85facc9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/177539
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>