Add check for minimal versions and fix compilation + more borrow

[Mingun]

While I was determining the actual minimum version of serde, I found the following:

• StrDeserializer::new was introduced in Add constructor function for all IntoDeserializer impls serde-rs/serde#2246 in serde v1.0.139 only. I've replaced it with BorrowedStrDeserializer::new, which is available since serde v1.0
• As a result of the previous change, $text and $value special fields now can be deserialized into borrowed fields, for example, into BTreeMap<&str, T>
• serde does not follow semver neither in version numbering neither in the specifying its dependencies to sibling crates (serde->serde_derive). As a result, cargo minimal-versions tries to downgrade serde to 1.0.100, but serde_derive to 1.0.0 which is not compiled. For example, serde_derive at least up to 1.0.60 depends on the syn v0.13 which does not have a used method make_where_clause, which was introduced only in syn v0.14. The other thing, that we are use in our tests, are #[serde(other) which are allowed not only in #[serde(field_identifier)] enums only since serde_derive v1.0.79. So I explicitly indicate dependency from serde_derive v1.0.79
• method fill_buf in tokio was introduced only since 1.10

[codecov-commenter]

Codecov Report

Merging #565 (04cc790) into master (05b754b) will increase coverage by 0.01%.
The diff coverage is 75.00%.

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@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   63.29%   63.30%   +0.01%     
==========================================
  Files          33       33              
  Lines       16479    16483       +4     
==========================================
+ Hits        10430    10435       +5     
+ Misses       6049     6048       -1     

Flag	Coverage Δ
unittests	63.30% <75.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/map.rs	83.93% <71.42%> (-0.19%)	⬇️
src/de/var.rs	90.62% <100.00%> (ø)
src/de/mod.rs	70.68% <0.00%> (+0.04%)	⬆️
src/reader/mod.rs	95.04% <0.00%> (+0.10%)	⬆️

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