Cache the span by using BufferWriter<T> within JsonWriter<T>

[ahsonkhan]

Borrows most of the changes from #2366, and supersedes #2358

• Utilizing BufferWriter to cache the span being written to and add the Flush method.
• Keeping a single type for now, as it will only support UTF-8 (will remove UTF-16 support later).
• Reducing the JsonWriter struct size by utilizing the highest bit of _indent for _firstItem bool.
• Fix typo in BufferWriter.Write(byte)

TODO: Add helper factory convenience methods.

Let me know if I missed anything. Any suggestions to reduce the overhead further are welcome :)

• ~10% regression for hello world (overhead of creating BufferWriter and no benefit from caching the span)
• ~7% improvement for writing larger json payload, say 10-50 elements (overhead amortized, benefit of caching span kicks in)
• ~20% improvement for writing large json payload, say 1000 array elements

cc @benaadams, @JeremyKuhne

BenchmarkDotNet=v0.10.14.534-nightly, OS=Windows 10.0.14393.2312 (1607/AnniversaryUpdate/Redstone1)
Intel Core i7-6700 CPU 3.40GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
Frequency=3328126 Hz, Resolution=300.4694 ns, Timer=TSC
.NET Core SDK=2.2.100-preview1-009015
  [Host]     : .NET Core 2.2.0-preview1-26609-02 (CoreCLR 4.6.26606.04, CoreFX 4.6.26606.04), 64bit RyuJIT
  DefaultJob : .NET Core 2.2.0-preview1-26609-02 (CoreCLR 4.6.26606.04, CoreFX 4.6.26606.04), 64bit RyuJIT

BEFORE (master):

Method	IsUTF8Encoded	Formatted	size	Mean	Error	StdDev	Allocated
WriterSystemTextJsonArrayOnly	True	False	1	83.15 ns	0.9320 ns	0.8262 ns	0 B
WriterSystemTextJsonArrayOnly	True	False	10	231.66 ns	4.6436 ns	6.0380 ns	0 B
WriterSystemTextJsonArrayOnly	True	False	1000	19,022.72 ns	369.7130 ns	467.5686 ns	0 B
WriterSystemTextJsonBasic	True	False	?	975.70 ns	19.5078 ns	43.2279 ns	0 B
WriterSystemTextJsonHelloWorld	True	False	?	104.12 ns	1.8728 ns	2.0039 ns	0 B

AFTER (this PR):

Method	IsUTF8Encoded	Formatted	size	Mean	Error	StdDev	Allocated
WriterSystemTextJsonArrayOnly	True	False	1	91.47 ns	2.301 ns	3.445 ns	0 B
WriterSystemTextJsonArrayOnly	True	False	10	214.79 ns	4.653 ns	6.673 ns	0 B
WriterSystemTextJsonArrayOnly	True	False	1000	15,995.46 ns	269.920 ns	239.277 ns	0 B
WriterSystemTextJsonBasic	True	False	?	908.57 ns	17.670 ns	21.035 ns	0 B
WriterSystemTextJsonHelloWorld	True	False	?	113.91 ns	2.311 ns	5.799 ns	0 B