cmd/compile/internal/types: simpler signature type representation

Now that all of the uses of signature types have been cleaned up, we
can simplify the internal representation significantly.

In particular, instead of 3 separate struct objects each with 3
separate slices of fields, we can store all of the parameters in a
single slice and track the boundaries between them.

We still need a results tuple struct for representing the type of
multi-value call expressions, but just a single one and it can safely
reuse the results subsection of the full parameters slice.

Note: while Sizeof(Func) has increased (e.g., 32->56 on amd64), we're
saving on the allocation of 2 Types, 2 Structs, and 2 []*Field (288
bytes total on amd64), not counting any extra GC size class padding
from using a single shared []*Field instead of 3 separate ones.

Change-Id: I119b5e960e715b3bc4f1f726e58b910a098659da
Reviewed-on: https://go-review.googlesource.com/c/go/+/521335
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Bypass: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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