Use string literals for ENCODED_NODE

[stepancheg]

Blocks like these:

                *b"\x00\x00\x00\x00\x00\x00\x00\x00",
                *b"schema.c",
                *b"apnp:Nod",
                *b"e.Nested",
                *b"Node\x00\x00\x00\x00",
                *b"\x00\x00\x00\x00\x01\x00\x01\x00",

give a clue what is inside.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.05%. Comparing base (ab342b3) to head (efcbadd).
⚠️ Report is 288 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/lib.rs	0.00%	13 Missing ⚠️
capnpc/src/pointer_constants.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #644      +/-   ##
==========================================
- Coverage   51.64%   51.05%   -0.60%     
==========================================
  Files          69       71       +2     
  Lines       33735    32501    -1234     
==========================================
- Hits        17422    16592     -830     
+ Misses      16313    15909     -404     

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