std lib: modify allocator idiom

Before we accepted a nullable allocator for some stuff like
opening files. Now we require an allocator.

Use the mem.FixedBufferAllocator pattern if a bound on the amount
to allocate is known.

This also establishes the pattern that usually an allocator is the
first argument to a function (possibly after "self").

fix docs for std.cstr.addNullByte

self hosted compiler:
 * only build docs when explicitly asked to
 * clean up main
 * stub out zig fmt

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