subscriber: use of SmallVec in EnvFilter makes them quite large

[hawkw]

@hawkw I think we are experiencing stack overflow on windows as a result of this update.
The previous size_of EnvFilter was 200, but now it is 5420.

It was hard, but we captured the backtrace on overflow on windows:

Details

  thread 'main' has overflowed its stack
  STACK OVERFLOW:
     0: backtrace::backtrace::dbghelp::trace
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.60\src\backtrace\dbghelp.rs:98
     1: backtrace::backtrace::trace_unsynchronized
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.60\src\backtrace\mod.rs:66
     2: backtrace::backtrace::trace<closure-0>
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.60\src\backtrace\mod.rs:53
     3: backtrace::capture::Backtrace::create
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.60\src\capture.rs:176
     4: backtrace::capture::Backtrace::new
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.60\src\capture.rs:140
     5: build_script_build::report_overflow
               at build.rs:54
     6: build_script_build::vectored_handler
               at build.rs:47
     7: RtlInitializeCriticalSectionAndSpinCount
     8: RtlWalkFrameChain
     9: KiUserExceptionDispatcher
    10: alloc::vec::spec_from_iter_nested::{{impl}}::from_iter<tracing_subscriber::filter::env::directive::Directive,core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::str::iter::Sp
lit<char>, closure-0>, tracing_subscriber::filter::directive::
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\alloc\src\vec\spec_from_iter_nested.rs:17
    11: alloc::vec::spec_from_iter::{{impl}}::from_iter<tracing_subscriber::filter::env::directive::Directive,core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::str::iter::Split<cha
r>, closure-0>, tracing_subscriber::filter::directive::ParseEr
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\alloc\src\vec\spec_from_iter.rs:33
    12: alloc::vec::{{impl}}::from_iter<tracing_subscriber::filter::env::directive::Directive,core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::str::iter::Split<char>, closure-0>,
tracing_subscriber::filter::directive::ParseError>>
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\alloc\src\vec\mod.rs:2449
    13: core::iter::traits::iterator::Iterator::collect<core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::str::iter::Split<char>, closure-0>, tracing_subscriber::filter::directive:
:ParseError>,alloc::vec::Vec<tracing_subscriber::filter::env::
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\core\src\iter\traits\iterator.rs:1748
    14: core::result::{{impl}}::from_iter::{{closure}}<tracing_subscriber::filter::env::directive::Directive,tracing_subscriber::filter::directive::ParseError,alloc::vec::Vec<tracing_subscriber::filter
::env::directive::Directive, alloc::alloc::Global>,core::iter:
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\core\src\result.rs:1625
    15: core::iter::adapters::process_results<core::iter::adapters::map::Map<core::str::iter::Split<char>, closure-0>,tracing_subscriber::filter::env::directive::Directive,tracing_subscriber::filter::d
irective::ParseError,closure-0,alloc::vec::Vec<tracing_subscri
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\core\src\iter\adapters\mod.rs:145
    16: core::result::{{impl}}::from_iter<tracing_subscriber::filter::env::directive::Directive,tracing_subscriber::filter::directive::ParseError,alloc::vec::Vec<tracing_subscriber::filter::env::direct
ive::Directive, alloc::alloc::Global>,core::iter::adapters::ma
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\core\src\result.rs:1625
    17: core::iter::traits::iterator::Iterator::collect<core::iter::adapters::map::Map<core::str::iter::Split<char>, closure-0>,enum$<core::result::Result<alloc::vec::Vec<tracing_subscriber::filter::en
v::directive::Directive, alloc::alloc::Global>, tracing_subscr
               at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633\library\core\src\iter\traits\iterator.rs:1748
    18: tracing_subscriber::filter::env::EnvFilter::try_new<alloc::string::String*>
               at C:\Users\Administrator\.cargo\registry\src\github.com-1ecc6299db9ec823\tracing-subscriber-0.2.22\src\filter\env\mod.rs:174

Not sure if this is a bug in particular, because opting out of smallvec feature fixes the problem, but still this has broken our builds =(

Originally posted by @Veetaha in #1550 (comment)

[hawkw]

Closed by #1568.

[Veetaha]

Wow, thanks for such a quick fix, that's pretty cool! =)