zig: update to zig/0.8.1

Adapt *.zig to changes in the zig library, and Makefile/run to the new
output path for executables.

Build system:
* avoid symbolic links to executables.  They increase complexity, are
  not useful, their timestamps or existence confuse make.
* Acknowledge that Makefile is unable to build each step separately.

Step files:
* Add commented instructions printing `ast` at the start of the EVAL
  loop.

The state in this commit now crashes with:
/lib/std/hash_map.zig:863:33: 0x20ea58 in std.hash_map.HashMap([]const
u8,*types.MalType,std.hash_map.StringContext,80).get (step6_file)
Build with release-safe until this segmentation fault is fixed

The original author has been contacted and intends to investigate.

impls/zig/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:bionic
+FROM ubuntu:20.04
 MAINTAINER Joel Martin <[email protected]>
 
 ##########################################################
@@ -11,23 +11,16 @@ RUN apt-get -y update
 # Required for running tests
 RUN apt-get -y install make python
 
-# Some typical implementation and test requirements
-RUN apt-get -y install curl libreadline-dev libedit-dev libpcre3-dev
-
 RUN mkdir -p /mal
 WORKDIR /mal
 
 ##########################################################
 # Specific implementation requirements
 ##########################################################
 
-RUN apt-get -y install gcc gdc ldc gpg wget
+RUN apt-get -y install ca-certificates curl gcc libc-dev libpcre3-dev libreadline-dev xz-utils
 
-RUN wget https://ziglang.org/download/0.5.0/zig-linux-x86_64-0.5.0.tar.xz && \
-    echo `pwd`  && \
-    tar -xf zig-linux-x86_64-0.5.0.tar.xz && \
-    cp -r zig-linux-x86_64-0.5.0 /usr/local/bin && \
-    ln -sf /usr/local/bin/zig-linux-x86_64-0.5.0/zig /usr/local/bin/zig && \
-    chmod +x /usr/local/bin/zig
+RUN curl https://ziglang.org/download/0.8.1/zig-linux-x86_64-0.8.1.tar.xz | tar -xJC/mal
+RUN ln -fst/usr/local/bin /mal/zig-linux-x86_64-0.8.1/zig
 
 ENV HOME /mal

impls/zig/Makefile

@@ -1,16 +1,16 @@
 
 STEPS = step0_repl step1_read_print step2_eval step3_env step4_if_fn_do step5_tco step6_file step7_quote step8_macros step9_try stepA_mal
 
-all: $(STEPS)
+# TODO: it would be nice to be able to rebuild only one step.
 
-dist: mal
+# safe until the segfault is fixed.
+# make HARD=1 REGRESS=1 test^zig
+# was absent from 1c76e857f8f40c46a9b08274a26a0cca7fb20c77 with zig/0.5.0
 
-
-%: %.zig
-	zig build -Drelease-fast=true
-	ln -sf zig-cache/bin/$* .
+all $(STEPS):
+	zig build -Drelease-safe=true
 
 .PHONY: clean
 
 clean:
-	rm -f $(STEPS)
+	rm -fr zig-cache zig-out