error: partially re-inline error constructors

[BurntSushi]

It turns out that #379 ended up pretty badly regressing datetime parsing
performance. In particular, the with_context routine on Result<T, E>
was eagerly calling the caller-provided closure even in the success
case. Which means error values (which are generally much more expensive
to build than to parse a datetime) were being constructed on every
parse. Owch.

I'm not sure how I missed this. I ran the benchmarks in #379 and they
didn't report anything egregious. I should double-back and improve
benchmark coverage.

Thankfully, this doesn't seem to regress the number of LLVM lines we're
emitting:

  Lines                 Copies              Function name
  -----                 ------              -------------
  192746                2456                (TOTAL)
    3978 (2.1%,  2.1%)     1 (0.0%,  0.0%)  jiff::fmt::strtime::format::Formatter<W,L>::format_one
    3785 (2.0%,  4.0%)     1 (0.0%,  0.1%)  jiff::fmt::strtime::parse::Parser::parse
    3600 (1.9%,  5.9%)     2 (0.1%,  0.2%)  jiff::fmt::temporal::printer::SpanPrinter::print_span
    3340 (1.7%,  7.6%)     2 (0.1%,  0.2%)  jiff::fmt::friendly::printer::SpanPrinter::print_duration_designators
    3250 (1.7%,  9.3%)    86 (3.5%,  3.7%)  <core::result::Result<T,jiff::error::Error> as jiff::error::ErrorContext>::with_context::{{closure}}
    2910 (1.5%, 10.8%)     6 (0.2%,  4.0%)  jiff::fmt::temporal::printer::DateTimePrinter::print_time
    2190 (1.1%, 12.0%)     1 (0.0%,  4.0%)  jiff::span::Span::from_invariant_nanoseconds
    2166 (1.1%, 13.1%)     6 (0.2%,  4.3%)  jiff::fmt::temporal::printer::DateTimePrinter::print_date
    1760 (0.9%, 14.0%)     2 (0.1%,  4.4%)  jiff::fmt::temporal::printer::SpanPrinter::print_duration
    1588 (0.8%, 14.8%)     2 (0.1%,  4.4%)  jiff::fmt::friendly::printer::SpanPrinter::print_span_hms
    1450 (0.8%, 15.6%)     2 (0.1%,  4.5%)  jiff::fmt::friendly::printer::SpanPrinter::print_span_designators_non_fraction
    1432 (0.7%, 16.3%)     4 (0.2%,  4.7%)  jiff::fmt::temporal::printer::DateTimePrinter::print_offset_rounded
    1398 (0.7%, 17.0%)     2 (0.1%,  4.8%)  jiff::fmt::temporal::printer::DateTimePrinter::print_pieces
    1368 (0.7%, 17.8%)     6 (0.2%,  5.0%)  jiff::fmt::friendly::printer::DesignatorWriter<W>::write
    1364 (0.7%, 18.5%)     2 (0.1%,  5.1%)  jiff::fmt::friendly::printer::SpanPrinter::print_duration_hms
    1248 (0.6%, 19.1%)    16 (0.7%,  5.7%)  <jiff::util::rangeint::RangedDebug<_,_> as core::fmt::Debug>::fmt
    1243 (0.6%, 19.8%)     1 (0.0%,  5.8%)  jiff::fmt::rfc2822::DateTimePrinter::print_civil_with_offset
    1222 (0.6%, 20.4%)    13 (0.5%,  6.3%)  jiff::util::rangeint::ri8<_,_>::new
    1108 (0.6%, 21.0%)    28 (1.1%,  7.5%)  core::result::Result<T,E>::unwrap
    1097 (0.6%, 21.5%)     1 (0.0%,  7.5%)  jiff::shared::tzif::Header::parse
    1086 (0.6%, 22.1%)    22 (0.9%,  8.4%)  core::option::Option<T>::map_or

Kudos to @charliermarsh for finding this while profiling uv.

[BurntSushi]

This PR is on crates.io in jiff 0.2.15.