Repo spring sweep

[phbnf]

Working on #225 revealed that the repo could do with some cleanups:

• remove go1.18 directives in tree_fuzz_test and node_fuzz_test.go
• bump the go version up
• add toolchain directive
• more idiomatic tests (see this PR comment)
• use range for loops were possible
• fix codecov which uses a very old base and keeps reporting error on CLs
• make reference inplementation of consistency and inclusion proof test signatures match RFCs