Add net.Buffers

[fgsch]

Should fix the remaining problem from mailru/easyjson#335, for the most part.

[fgsch]

For completion, if we try to build or run the code in the linked issue under macos we get:

Undefined symbols for architecture x86_64:
  "_crypto/x509/internal/macos.x509_CFStringCreateWithBytes_trampoline", referenced from:
      _crypto/x509/internal/macos$pack in main.o
  "_internal/abi.FuncPCABI0", referenced from:
      _crypto/x509/internal/macos.StringToCFString in main.o
  "_crypto/ed25519/internal/edwards25519/field.feSquare", referenced from:
      _(*crypto/ed25519/internal/edwards25519/field.Element).Square in main.o
  "_crypto/ed25519/internal/edwards25519/field.feMul", referenced from:
      _(*crypto/ed25519/internal/edwards25519/field.Element).Multiply in main.o
  "_vendor/golang.org/x/sys/cpu.xgetbv", referenced from:
      _runtime.run$1$gowrapper in main.o
  "_crypto/x509/internal/macos.syscall", referenced from:
      _crypto/x509/internal/macos.StringToCFString in main.o
  "_vendor/golang.org/x/sys/cpu.darwinSupportsAVX512", referenced from:
      _runtime.run$1$gowrapper in main.o
  "_vendor/golang.org/x/sys/cpu.cpuid", referenced from:
      _runtime.run$1$gowrapper in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: failed to link /.../tinygo1105991511/main: exit status 1

This builds and runs fine under wasi, and at least builds in other platforms.

[dgryski]

This is because of the net/http import in easyjson's helpers.go.

[dgryski]

LGTM. Are there net.Buffers tests from stdlib that should be moved over too?

[fgsch]

@dgryski there are some we can use. I will update this PR tomorrow.

[deadprogram]

@fgsch will you add the tests you mentioned to this PR? 😸

[fgsch]

@deadprogram Yeah, sorry. Been afk the last couple of days. I will get this done today.

[fgsch]

@deadprogram tests added.

[deadprogram]

Thank you @fgsch for working on this and thanks @dgryski for the review. Now merging.