feat: add fntype

[Chronostasys]

close #283

This pr only considered pure function situations. How to handle real closure values is leaved for future investigaion.

Use generic function as right value is also disallowed, probably need some modification in TypeNameNode to support it. I'll add more details in issue #284

[codecov]

Codecov Report

Merging #285 (659ba29) into master (acc8782) will decrease coverage by 0.02%.
The diff coverage is 90.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
- Coverage   87.50%   87.49%   -0.02%     
==========================================
  Files          99       99              
  Lines       17080    17366     +286     
==========================================
+ Hits        14946    15194     +248     
- Misses       2134     2172      +38     

Impacted Files	Coverage Δ
src/ast/node/mod.rs	82.53% <ø> (ø)
src/ast/node/ret.rs	87.03% <ø> (ø)
src/ast/ctx.rs	89.08% <47.82%> (-1.67%)	⬇️
src/ast/diag.rs	61.32% <66.66%> (+0.06%)	⬆️
src/ast/pltype.rs	86.15% <68.00%> (-1.03%)	⬇️
src/ast/node/function.rs	86.55% <82.31%> (+1.19%)	⬆️
src/nomparser/types.rs	98.68% <94.73%> (-0.37%)	⬇️
src/ast/test.rs	95.71% <95.60%> (-0.01%)	⬇️
src/ast/builder/llvmbuilder.rs	90.90% <100.00%> (+0.19%)	⬆️
src/ast/fmt.rs	89.23% <100.00%> (+0.21%)	⬆️
... and 3 more

... and 1 file with indirect coverage changes

[github-actions]

Benchmark for 12efb80

Click to view benchmark

Test	Base	PR	%
multithreads(2) gc benchmark--65535 small objects(per thread)	1385.8±35.90µs	1639.6±510.41µs	+18.31%
plimmixgc/multi-thread gc stress benchmark small objects	1149.2±28.40ms	1140.8±34.88ms	-0.73%
plimmixgc/singlethread gc stress benchmark small objects	1064.5±24.94ms	1059.9±23.56ms	-0.43%
singlethread gc benchmark--65535 small objects	1285.4±9.96µs	1269.7±10.17µs	-1.22%
singlethread gc mark benchmark--65535 small objects	744.7±10.77µs	730.9±8.22µs	-1.85%
singlethread gc sweep benchmark--65535 small objects	544.9±2.97µs	544.0±1.42µs	-0.17%

[github-actions]

Benchmark for 54bf090

Click to view benchmark

Test	Base	PR	%
multithreads(2) gc benchmark--65535 small objects(per thread)	1731.4±210.69µs	2.1±0.33ms	+21.29%
plimmixgc/multi-thread gc stress benchmark small objects	1244.5±34.80ms	1227.7±42.72ms	-1.35%
plimmixgc/singlethread gc stress benchmark small objects	1123.6±19.06ms	1130.8±19.95ms	+0.64%
singlethread gc benchmark--65535 small objects	1455.0±121.81µs	1506.5±120.68µs	+3.54%
singlethread gc mark benchmark--65535 small objects	900.9±69.93µs	872.0±78.91µs	-3.21%
singlethread gc sweep benchmark--65535 small objects	583.2±50.67µs	565.1±28.92µs	-3.10%

[RINNE-TAN]

LGTM