feat: add ability to reset so the object can be reused

[muktihari]

Now Decoder and Encoder have the ability to Reset, the underlying storage will be reused reducing memory allocation.

benchmark:

goos: darwin
goarch: amd64
pkg: github.com/muktihari/fit/decoder
cpu: Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
BenchmarkReset/benchmark_New()-4          419300        2475   ns/op      9632 B/op       6 allocs/op
BenchmarkReset/benchmark_Reset()-4       6264751         207.6 ns/op       120 B/op       3 allocs/op

goos: darwin
goarch: amd64
pkg: github.com/muktihari/fit/encoder
cpu: Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
BenchmarkReset/benchmark_New()-4          1000000      1155   ns/op       2283 B/op      13 allocs/op
BenchmarkReset/benchmark_Reset()-4        6866968       168.6 ns/op         97 B/op       3 allocs/op

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c4128ca) 99.89% compared to head (23b3ec6) 99.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   99.89%   99.90%           
=======================================
  Files          43       43           
  Lines        3989     4038   +49     
=======================================
+ Hits         3985     4034   +49     
  Misses          4        4           

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