feat: Add public NewFromDecimal which outperforms decimal -> string -> alpacadecimal

[aarjaneiro]

Should close #7

Note, from my benchmarking:

goos: linux
goarch: amd64
pkg: github.com/alpacahq/alpacadecimal
cpu: AMD Ryzen 9 3900XT 12-Core Processor           
BenchmarkNewFromDecimal
BenchmarkNewFromDecimal/alpacadecimal.Decimal.NewFromDecimal
BenchmarkNewFromDecimal/alpacadecimal.Decimal.NewFromDecimal-24         	47093888	        25.64 ns/op
BenchmarkNewFromDecimal/alpacadecimal.Decimal.RequireFromString
BenchmarkNewFromDecimal/alpacadecimal.Decimal.RequireFromString-24      	 5173086	       233.9 ns/op
BenchmarkNewFromDecimal/alpacadecimal.Decimal.New
BenchmarkNewFromDecimal/alpacadecimal.Decimal.New-24                    	507679633	         2.316 ns/op
PASS

Where NewFromDecimal is the added method, RequireFromString implements the conversion method specified as slow in the linked issue, and New simply creates the alpacadecimal.Decimal in-place (no conversion from decimal.Decimal).