patched_stdlib(parsejson): resolve CStringConv warnings

[ee7]

After the upgrade to Nim 1.6.0 in commit 761936b, a regular
nimble build would produce these warnings:

/foo/configlet/src/patched_stdlib/parsejson.nim(237, 37) Warning: implicit conversion to 'cstring' from a non-const location: my.buf; this will become a compile time error in the future [CStringConv]

/foo/configlet/src/patched_stdlib/parsejson.nim(247, 39) Warning: implicit conversion to 'cstring' from a non-const location: my.buf; this will become a compile time error in the future [CStringConv]

I authored a commit in the nim-lang/Nim repo that resolves the
most visible CStringConv warnings, including these in parsejson, by
making the cstring conversion explicit. Let's update configlet's copy of
parsejson without waiting for a Nim version bump, so that we no longer
see these CStringConv warnings while compiling configlet.

As a reminder, we maintain our own copy of std/json and std/parsejson in
the configlet repo so that we can forbid comments and trailing commas in
JSON (see 15c8403 for details).

As background, the Nim manual says that an implicit conversion to
cstring will eventually not be allowed:

A Nim string is implicitly convertible to cstring for convenience.

[...]

Even though the conversion is implicit, it is not safe: The garbage collector
does not consider a cstring to be a root and may collect the underlying
memory. For this reason, the implicit conversion will be removed in future
releases of the Nim compiler. Certain idioms like conversion of a const string
to cstring are safe and will remain to be allowed.

And from Nim 1.6.0, such a conversion triggers a warning:

A dangerous implicit conversion to cstring now triggers a [CStringConv] warning.
This warning will become an error in future versions! Use an explicit conversion
like cstring(x) in order to silence the warning.

---

To-do:

• Resolve this upstream, and then I can update the hash in our parsejson.nim file.

[ErikSchierboom]

@ee7 Can this be merged/updated? Those warnings are annoying :)

[ee7]

I'll try to finally finish nim-lang/Nim#19488 soon. I have a minor preference for merging this PR after that, so that we can either:

1. Update the hash below and keep the diff from upstream as small as possible:

configlet/src/patched_stdlib/json.nim

Lines 1 to 5 in 85e262c

	# This file is minimally adapted from this version in the Nim standard library:
	# https://github.com/nim-lang/Nim/blob/c17baaefbcff/lib/pure/json.nim
	# The standard library version is lenient: it silently allows a trailing comma.
	# The below version is stricter: it raises a `JsonParsingError` for a trailing
	# comma.

2. or at least reference the upstream commit in our commit message, if we'll base our patched file on version-1-6 rather than Nim devel.

[ee7]

I finally got around to finishing the upstream PR. So hopefully that'll be merged soon, and I'll finally merge this PR.

Sorry for the wait.

[ee7]

I fixed these warnings in the repo for the Nim compiler, and rebased our patched parsejson on Nim devel.