This is a bigger PR. I am sorry. It started off with parsing, but then got into printing too. If it's too large to review, I will try to split it into multiple PRs.

Summary:

• Parsing implemented according to https://speleotrove.com/decimal/daconvs.html#refnumsyn
• tryparse is now available too
• Printing implemented via scientific notation described at https://speleotrove.com/decimal/daconvs.html#reftostr
• Removed number(::Decimal): the function was type-unstable, returning Float64 or Int64 depending on the value of the Decimal. The user has the option to call Float64(::Decimal) or Int64(::Decimal) based on their need.
• Removed decimal(x): The user can use Decimal(x) if x is a number, and parse(Decimal, x) or (newly) dec"123.456" if x is a string.

Fixes #20:

julia> parse(Decimal, "3.2.1")
ERROR: ArgumentError: Invalid decimal: 3.2.1

Fixes #45:

# Again, decimal(::String) removed, but using parse/dec_str:
julia> parse(Decimal, "1.23456789e-13")
1.23456789E-13

julia> parse(Decimal, "1.23456789e-14")
1.23456789E-14

julia> parse(Decimal, "1.23456789e-15")
1.23456789E-15

julia> parse(Decimal, "1.23456789e-21")
1.23456789E-21

Fixes #53:

julia> x = 1.2345678e6
1.2345678e6

julia> Decimal(x)
1234567.8

Closes #21
Closes #24
Closes #55