parser: Improve TOML frontmatter parser performance

parser: Improve TOML frontmatter parser performance

Difference between toml.Load(string(datum)) and
toml.LoadReader(bytes.NewReader(datum)):
benchmark           old ns/op     new ns/op     delta
BenchmarkLoad-4     82068         78489         -4.36%

benchmark           old allocs     new allocs     delta
BenchmarkLoad-4     494            493            -0.20%

benchmark           old bytes     new bytes     delta
BenchmarkLoad-4     17009         16913         -0.56%