ConstReadonlyArrayFixlen: 5Array of fixed-length values (fp32 / fp64 only).
ReadonlyArraySigned: 4Array of signed (zig-zag) varints.
ReadonlyArrayUnsigned: 3Array of unsigned varints.
ReadonlyFixlen: 2Fixed-length value: fp32, fp64, string or blob (see FixlenSubtype).
ReadonlySequenceEnd: 7Closes the current sequence. Encoded as the single byte 0x07.
ReadonlySequenceStart: 6Opens a nested sequence (new id scope).
ReadonlySigned: 1Signed varint scalar (zig-zag encoded).
ReadonlyUnsigned: 0Unsigned varint scalar.
The three low bits of a field header: what kind of field follows.