make Encoder/Decoder instances easily reusable

[gfx]

It changes the interfaces to Encoder and Decoder, so I'll bump the major version on the next release.

Benchmark Results

npx ts-node benchmark/benchmark-from-msgpack-lite.ts
Benchmark on NodeJS/v14.8.0 (V8/8.4)

operation                                                         |   op   |   ms  |  op/s
----------------------------------------------------------------- | ------: | ----: | ------:
buf = Buffer.from(JSON.stringify(obj));                           |  763800 |  5000 |  152760
buf = JSON.stringify(obj);                                        | 1086600 |  5000 |  217320
obj = JSON.parse(buf);                                            | 1634600 |  5000 |  326920
buf = require("msgpack-lite").encode(obj);                        |  462900 |  5000 |   92580
obj = require("msgpack-lite").decode(buf);                        |  271700 |  5001 |   54329
buf = require("@msgpack/msgpack").encode(obj);                    |  802400 |  5000 |  160480
obj = require("@msgpack/msgpack").decode(buf);                    |  724400 |  5000 |  144880
buf = /* @msgpack/msgpack */ encoder.encode(obj);                 | 1083000 |  5000 |  216600
obj = /* @msgpack/msgpack */ decoder.decode(buf);                 |  737500 |  5000 |  147500```

[codecov-commenter]

Codecov Report

Merging #121 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   98.46%   98.49%   +0.02%     
==========================================
  Files          15       15              
  Lines         914      929      +15     
  Branches      196      198       +2     
==========================================
+ Hits          900      915      +15     
  Misses         14       14              

Impacted Files	Coverage Δ
src/CachedKeyDecoder.ts	100.00% <100.00%> (ø)
src/Decoder.ts	99.14% <100.00%> (+0.01%)	⬆️
src/Encoder.ts	97.97% <100.00%> (+0.05%)	⬆️
src/decode.ts	100.00% <100.00%> (ø)
src/decodeAsync.ts	100.00% <100.00%> (ø)
src/encode.ts	100.00% <100.00%> (ø)

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