[#255] Add mapKeysWith and camelCaseKeys

[jacob-alford]

Closes #255

Introduces a powerful type operator and utility function for transforming a struct definition's output keys with a mapping function. And adds a camelcase keys utility.

import * as E from 'fp-ts/Either'
import { getDecoder } from 'schemata-ts/Decoder'
import * as S from 'schemata-ts/schemata'
import * as s from 'schemata-ts/struct'

const camelFromMixed = s.camelCaseKeys(
  s.defineStruct({
    foo_bar: s.required(S.String),
    'bar-qux': s.optional(S.Number),
    'foo.baz-dan': s.optional(S.Boolean),
  }),
)

const DecoderMapMixedToCamel = getDecoder(S.StructM(camelFromMixed))

assert.deepStrictEqual(
  DecoderMapMixedToCamel.decode({
    foo_bar: 'foo',
    'bar-qux': 1,
    'foo.baz-dan': true,
  }),
  E.right({ fooBar: 'foo', barQux: 1, fooBazDan: true }),
)

[coveralls]

Pull Request Test Coverage Report for Build 4194677949

• 67 of 67 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 4179116618:	0.0%
Covered Lines:	2307
Relevant Lines:	2307

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💛 - Coveralls